I have heard that one floats and the other doesn't. As far as stretching, unfortunately semen tends to stretch some as well, so unless you have been checking for a while it is quite hard to distinguish.

I think FF basically analyses the data based on what you enter, if you enter EWCM then it will take it as that, not a possibility of semen. When I first started temping I used to check CM, and CP but didn't include it on my chart until such time as I could distinguish the two.

It is hard, but it does get easier, have you done the course that FF offers? They send it daily to your email and may help answer some questions