Heather was always amazing. Real-life couples have more credibility when they claim the guy is shooting down her throat, compared to a commercial series. Fake creampie series are all-too-common, so I don't think there's much faith in series that claim the guy is cumming without evidence of any sort.
But, if they were smart, I think it's possible. The trick is to have the girl backwards so that you can see her face and the underside of his cock at the same time, and then try to film his cock convulsing as he's ejaculating down her throat. The other indirect evidence is if he claims to cum and quickly goes limp, but that's unpredictable.
I think the key is seeing the cock spasms from pumping cum... it's actually a cool visual, both for deep throats and for creampies.