If you are asking this question, something specific probably happened. They mentioned a place you did not tell them about. They knew when you left work. They referred to a conversation you are certain they were not part of.

Here is the thing most articles get backwards. People who ask this question are usually right that something is going on, and usually wrong about what. They picture hidden software. What they almost always find is a setting.

Start with the boring answer, because it is usually the right one

Before you look for spyware, check the features designed to share your location. They are the most common reason people are correctly tracked, they need no technical skill, and they are invisible until you go looking.

On iPhone

Settings, then your name, then Find My, then Share My Location. Anyone listed there sees where you are, continuously.
Settings, then your name, then Family Sharing. Family members may see location, purchases and screen time.
Settings, then Privacy and Security, then Location Services, then Share My Location. Check which devices are listed.
In the Find My app, open the People tab. This is the clearest list of who can see you.

On Android

Google Maps, tap your profile picture, then Location sharing. Anyone listed sees you in real time.
Google Family Link, if it is set up, gives a family manager location and app control.
Settings, then Google, then Find My Device, and check your account at myaccount.google.com for devices you do not recognise.
Samsung phones add SmartThings Find; check it separately.

Both

Shared calendars. A shared calendar reveals where you will be, which is often more useful to somebody than where you are.
Shared photo albums with location data attached to the images.
Item trackers. An AirTag or Tile in a bag or car tracks you without touching your phone. iPhone alerts you about unknown AirTags automatically; on Android, Google's unknown tracker alerts do the same, and Apple publishes a Tracker Detect app for Android.

If you find something here, you have your answer, and it is the easiest kind to fix.

Before you switch anything off

If the person you suspect is a partner, an ex, a family member or an employer, pause here.

Turning off location sharing is visible. Find My tells people when sharing stops. So does Google Maps. In a controlling situation, flipping that switch is a message, sent at a moment you did not choose, and it can escalate things.

That does not mean leave it on. It means decide deliberately, and ideally after talking to somebody who handles these situations daily. The organisations at the end of this article are free and confidential. Use a device the other person has never had access to.

There is a middle path worth knowing about: you can often make your location less useful without making a change that gets announced. Leaving the phone somewhere, or travelling without it, tells you a great deal about whether you are being watched, and tells them nothing.

When it is not a setting

If the sharing features are clean and the feeling persists, work through the real detection checks.

The account route. Someone with your Apple ID or Google password does not need anything on your phone. They can see your location, read your backups, and see your messages from their own device. Check your account's device list, and check the recovery email and phone number on the account — a recovery address belonging to the other person hands the account back no matter how often you change the password.

Monitoring software. On Android, look at Settings, Accessibility (downloaded services), notification access, and device admin apps. On iPhone, look at Settings, General, VPN and Device Management. Our detailed walkthroughs are detecting spyware on Android and detecting spyware on iPhone.

The car, not the phone. Modern cars have connected apps that report location to whoever set up the account. If the car was bought or registered by somebody else, that is a tracking channel nobody thinks to check.

What does not prove anything

Ads that match a conversation. This is the single most common reason people conclude their phone is listening. The honest explanation is almost always simpler and stranger: advertising networks know who you spend time with, what they search for, and where you both were. You do not need to be recorded to be predicted.
Battery and heat. An ageing battery produces exactly the same symptoms. Worth noticing, never conclusive on its own.
Codes people share online. Dialling sequences that supposedly reveal tracking mostly show call-forwarding settings, which is a different thing entirely. We go through those in signs of a tapped phone.

How to know for sure

The uncomfortable truth is that no check you run on the phone can fully clear it, because a compromised phone is being asked to report on itself.

Analysis from a computer does not have that problem. Connected by cable, the phone produces a backup and diagnostic data that no app on the phone is allowed to read, and the computer draws its own conclusions from it. That is the method the researchers who publish this work actually use, and it is what Haris Shield does: scan free, with a readable report if you need one for a lawyer, an officer or a support worker.

If you want it to stop

The order matters more than the steps.

Get advice first if somebody in your life is involved, from a device they have never touched.
Document before you change anything. Screenshots of sharing settings, and a forensic report if there is software involved.
Secure the accounts, from a different device, email first. Check recovery addresses and signed-in devices as you go.
Then turn off the sharing, knowing it may be noticed, ideally with support in place.
Physical access last but not least. A passcode nobody knows, and no extra fingerprints or face entries. Everything above is reversible by someone who can pick up your unlocked phone.

Where to get help

Coalition Against Stalkerware: stopstalkerware.org
NNEDV Safety Net, technology safety planning: techsafety.org
Refuge (UK), tech abuse support: refuge.org.uk

If you are in immediate danger, contact your local emergency number.

The short answer

Check the sharing features first: Find My, Family Sharing, Google Maps location sharing, Family Link, shared calendars, item trackers. That is the answer far more often than spyware is, and it takes five minutes.

If those are clean, check the account route and then the monitoring-software screens. And if somebody in your life may be behind it, get advice before you switch anything off, because switching it off is visible.

Sources

Apple, Share your location with family and friends: support.apple.com/en-us/104996
Google, Share your real-time location: support.google.com/maps/answer/7326816
Coalition Against Stalkerware, Information for survivors: stopstalkerware.org/information-for-survivors
Electronic Frontier Foundation, Surveillance Self-Defense: ssd.eff.org

Is Someone Tracking My Phone? How to Know for Sure