Opponent disconnections

I do that sometimes, more so when playing black.
I click on my opponent's screen name to see the little box of information, and may also click again to open the profile to quickly look at it. Just to see who and from where my opponent calls home and if they wrote about themselves. Also if I'm trying to decide on one of my opening options, depending on white's opening move, I will have to decide what and how I'm going to play. For instance am I going to fianchetto my bishop or both bishops, where and how i'm going to play my center pawns and knights (not to be in the bishop's diagonal play way) etc.
I'm aware what you're alluding to...that maybe they are consulting an engine or opening book. Don't worry about it, if you have a reasonable suspicion of a fair play violation. Report it. There's no way for you to tell for sure on your own. But chess.com has a very sophisticated way of checking complaints. No penalty to you if you're wrong, just don't report every loss.
Another suggestion join the "Cheating Forum" club. They discuss and post about cheating, use of engines, etc. It's how I educated myself about it, reading the posts & notes. Read the rules for the forum before you post, to not run afoul of their strict rules (no public naming & shaming, or posting game links where the opponent can be identified by name, rating, etc.).