Usually it doesn't matter. If you are with random people, it doesn't matter because random ... form a group if you don't want that chance. PuGs formed through chat, it still doesn't matter because you still can't verify realistically if it was (PC/ISP) lag or not.
If you are with people you know; lag can be usually one of three things; internet latency, system hangup, mistake. If they are your friend irl, it still doesn't matter, if they are a decent 'internet friend' it still shouldn't matter. You'll know them well enough to figure out the truth of the situation.
Internet lag happens. PC hardware lag happens. Mistakes happen.
I've never had it become a real issue before. I've never had anyone consistently pull that excuse week after week. I've had people have a bad night with either their system or ISP and it clear up. I've had people have a bad night where they just can't pull it together for whatever reason that are normally consistent.
Don't take the game that seriously, and quit being an antagonist looking for any reason to jump down someone's throat. It will not get you far in life, even if it does temporarily make up for your lack of self-esteem by feeling like an internet badass pointing out the obvious or calling someone out who might have some esteem issues of their own but don't try to overcompensate by going aggressive.
Just. Have. Fun.