I probably have been fooled many times but in my opinion what looks fake is when the cum is too homogenous and sticky. I know according to Paige Jennings/Veronica Vain that they use a Cetaphil product in the US. Holly Randall said she had female performers who insisted in getting real cum in mouth and not fake because of the sensation and taste. Think it was Adriana IIRC.
I think what makes it real is when you can see the dick muscles pump. Maybe it is different for different guys but my own dick muscles cramp like hell and I could shoot a load at least a meter or soo. Some guys just have dripping ejaculations. My current favorite is John Price who can do like five to eight good shots and distribute among several girls. Great!
This one looks fake to me.

DAP-addicts post shows something which looks real.
BTW lubrication is important. I like to take a hand full and totally smear the ass and pussy. Then I take the bowling grip with the thumb in her ass and two fingers rubbing her pussy. Very effective. I don't know why we see so little of that. She likes it and I like it. Then I alternate with fucking and ass licking. Unfortunately many lubricants taste awful and chemical. I have tried different and some are good but lubricates less. For taste, coconut oil is the best, but you need to warm it in the microwave first,