Red means its a go
Just during or moments after orgasm many women, and a few men I have been told, display a bright red flush (or is it blush) on their upper chest, between about their neckline and their nipples. It is, of course, most obvious on fair skin. It usually fades within a minute or two, maybe less. Sometimes somewhat similar flushes are seen elsewhere, such as in the crotch or inner thighs. Not all women have this "symptom", and even those that do may not always have it.
Nevertheless, I suspect it must be quite hard to fake (maybe yogi masters could do it), so if you see a suddenly red chest, you can be pretty sure it is the real thing.
-Ww