Phone Hacked

There is no one way.  Each exploit has its own set of details.  Some are very good at hiding their existance.  So the best advice is to make an assumption. 

It’s pretty unlikely that your phone is hacked if you only downloaded apps from the Apple or Google stores.  If you have only installed apps from those stores you are probably OK.

On the other hand, if there are symptoms, such as lots of ads popping up, unexplained charges on your phone bill or credit card, or a sudden slow down, then you might assume that it has been infected with malware.

Fortunately, most malware won’t survive a factory reset.  Follow the procedures for your phone to preserve your data and perform a reset.  Then, you will KNOW that your phone is free of malware (for a while.)

OK, let’s roll….and Hey!...Lets be careful out there.

As @philesterhouse says, many different types of attacks.  Edward Snowdon has an interesting video, discussing some of the NSA toolkit attacks (listening and viewing while making the phone appear to be off for example) but points out that the vast majority of people are not going to subject to these types of attacks.

If someone such as a spouse has physical access to your phone, then certainly spyware of varying sophistication can be used.   But again, you would hopefully be aware if that was a possibility! 

The more common, as referenced by @philesterhouse would be stuff spamming you with ads and/or trying to steal credit card/passwords etc.   Various scanning programs offer some protection against these types of things.