Often
The checking happens: the phone, the followers, the details of an evening, the account matched against what took place. The first thing worth knowing is that this is the commonest form jealousy takes and, statistically, the most ordinary; the question "is it normal to check your partner's phone" gets asked as often as it does precisely because a great many people do. The useful part is elsewhere. Checking has a property that keeps it from arriving where it was aimed: finding nothing does not close it. An absence of findings reads not as "all is well" but as "not yet" — and the cycle continues regardless of the result. And the second thing, visible only from outside: of the three sides this is the only one that reaches the other person. Whatever stands behind it, what they receive is always the same message.
Sometimes
You don't set out to find anything, but you don't look away either. A screen lighting up nearby will get read; a question about an evening will occasionally run more detailed than it needed to; you will glance at your partner's feed with no design on it whatsoever. This is how most people live, and roughly speaking the line falls at effort: what came to you came to you, but you won't go looking. One thing is worth knowing about this middle. It moves with circumstances far more than with character: a stretch where trust has thinned lifts it quickly, and people usually notice not during the rise but afterwards — by finding that something they never used to do has become ordinary.
Rarely
No checking. A phone left on the table stays where it is; verifying an account of an evening doesn't occur to you; you don't go looking through anyone's follower list. Of the three sides this is the only one made entirely of actions rather than inner life, which is why it is the only one the other person can see. Hence its peculiarity: a low level here is read by a partner directly and without translation, as trust — even when your mind is meanwhile occupied with anything at all. The reverse holds too, and is worth knowing: with high thoughts and low checking, you are paying for someone else's peace of mind with your own restraint, and the price of that restraint is visible to nobody.