It feels different to me.. This might be TMI, but if you touch it between your fingers and its slippery and stretchy or watery, then its probably CM, if it feels feel filmy or sticky, especially when it starts to dry, then its probably semen. it just feels different to the touch to me.
I've learned that if it comes out in a big slimy glob the morning/day after BD'ing it's usually semen. It also has a much different smell. (no i don't stick my nose in it but it's an obvious smell) For me, CM is NEVER that abundant.
They're so similar that I couldn't tell you honestly. I get confused about that myself.