just talk it out i guess? if they're stressed out but don't want to say anything to you about it then the only choice is to approach them first and ask what's up. i suppose that'd ease the stress bc it shows you're taking an interest and care.
I'd probably leave them be. Of course, communication is good, but not with the kind of person I'm usually attracted to. My last ex would just stop talking to me when something was going on, and trying to communicate would get us absolutely nowhere.
I know nothing about relationships do not listen to me.