Yeah, "external" cum-shots i.e. on the face, belly, ass etc. are more likely to be real or at least to be faked better because they are easier to spot: with cream-pies be it inside the pussy or mouth, you obviously can't see through the skin, and in case of oral shots (even if it's an open-mouth oral from the distance, i.e. the girl opens her mouth, and the guy shoots inside it without touching her), you as the maker can also justify the "unusual" texture, color etc. by saying that it got mixed up with saliva etc. With external shots, there are fewer tricks you can play, and on the face especially, the cum will be under more inspection because we are just more used to be attentive to other's faces
Also, external ones and the open-mouth shots are just more difficult to fake technically since you'll need to either make the camera look away (which is blasphemy in case of facials since if any kind of cum-shot process needs to be shown it's the facial) or put the tubes in such a way that they are not visible. And tubes are more difficult to utilize and hide than just smearing your cock with white stuff just before the "oral cream-pie" while the camera looks away
Right, I got tired of writing about all the crap they do just not to use real cum. To rephrase a certain British comedy show, "Real cum? Bollocks more like". Well, at least we can be sure that, even if the are no raw insertions into the pussy/ass (condoms all the time), even if the cum is fake, we can take at least some creepy joy in the fact that the girls still need to touch the dick and the balls* directly with their lips and mouths. And being a guy mostly interested in oral in porn, that might be enough already
* I tried to include them both in some kind of a euphemism, but which one? I could've said "skin flute" but what about the balls then? I could've tried to use "bagpipe" even, but it's more like "the pipe and the pipe-bags"... So, just "dick and balls"