Well, here's how you tell (for Natorock and Flipper)!
I don't think I've ever confused leftover semen for EWCM.
Seminal fluid has a distinct odor (DF's is kind of like bleach). My CM does
not. Yeah, it's kind of weird to sniff things but hey, if there's any question about it, that's the easiest way.
Semen is much less stretchy ... at least DF's is. I'm not able to pull my fingers apart like with EWCM. A little, yes, but it's more like sticky CM as far as the stretchiness goes.
My EWCM is a little cloudy (yours could be clear or tinted or something else though). DF's semen is a lot whiter. Next time you BD see what his semen looks like (you could just collect some on a finger after he's done so as not to waste a whole BDing session) and then remember that for next time you're looking at EWCM.
Everything that goes in (in my experience) comes out much waterier than it went in. So when I check CM after BDing I think I'm much more likely to confuse semen for
watery CM than for eggwhite. That is, if I wait at least an hour or so (and why would you check it before that?

). Try to get some out and see if it's waterier coming out. I bet it is.
Semen will dissolve in water. CM will not.
If you check all that and you
still can't tell, mark it as EWCM. There is no point in missing a possibly fertile day just because you're not sure whether you're right about the cervical fluids.