How can I check if there are hidden third party apps on my iPhone?

If you feel an unauthorized person/app is remotely using, controlling or monitoring your device, then that is possible only if you have done one or more of the following Don'ts...

1. Don't hand over an iPhone to kids or to a stranger without Enabling Guided Access
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device
5. Don't give in to Phishing
6. Don't plug in your device in Airports and Public places through third-party cables and trust the device. Beware of Juice Jacking. (Especially in India)
7. Don't leave your iPhone unlocked and unattended in public places like offices, schools, malls, etc.

If one of the above is true then quickly change the Apple ID Password and Return iPhone settings to their defaults.

You cannot hide an App on an iPhone.

Probably some pages may be hidden

Here's how to hide/unhide a page:

1. Touch and hold an empty area on your Home Screen.
2. Tap the dots near the bottom of your screen.
3. Tap and Tick the circle under the page that you want to unhide.
4. Tap Done.

Please see the pic below which is self-explanatory

You may --> Organize the Home Screen and App Library on your iPhone