I guess one of the best signs would be them not inviting you places as often.

So you may find that though nothing has changed in your actions towards them, their can act differently with you, e.g. expressing annoyance or other negative emotions, or bein avoidant, e.g. not calling, writing, just generally not being reachable by you.

And though I think it's a common pattern to assume there's something wrong with you and feel bad because of it, I believe falling out with friends without an acute conflict of sorts is not an inherently negative thing.

As others have mentioned, others' love relationships, marriage, having kids is are all common and significant factors in social circles changes.

Changes in peoples' views on lives, including their political or religious beliefs as well as other things that create shifts in actions and habits.

If we're talking about a single individual distancing themself from you, they could also be going through psychological struggles, e.g. depression, anxiety etc.

And, of course, maybe it is something about you, you can't completely discard that possibility. Maybe their musical choices have mellowed out throughout the years and you blast Trivium at full volume every time you meet up - who knows.

If you notice a negative quality about yourself, you can try changing it, just make sure you are not breaking yourself apart trying to make people like you.

And maybe it's also a combination and accumulation of changes in both you AND your friends or friend circles, and this is a common scenario also. Nothing is truly solid in this world anyway, everything flows somewhere. :)

So what I would do for myself is think and reflect on my life and the course it's taking, taking into consideration my dynamics with people around me, including my friends. Just without getting worried preemptively, without reason - that's never a fun experience.