Depends on what you mean by "make a pass."

If it involves sexual touching, the other person has to consent first, otherwise it certainly is sexual assault.  

Maybe I understand what you're getting at.  In social situations (not the workplace) people will often negotiate consent nonverbally.  We move in for a kiss very slowly and incrementally.  Then the other person will either signal that they want to kiss you too, or keep their distance if they don't.  That's not a problem.  That's just asking for consent without using words.

But if you are getting mixed signals -- if you are unsure -- you can either drop it, wait to see what happens, or use your words.  That's how we avoid making "mistakes."  It's just plain wrong to force yourself on somebody.  I suspect you know that.