I've had my share of fake O's for sure from the WLs. I have one favorite who during our first few sessions she convinced me she was orgasming, but as I spent more time with her I learned her pattern and while it was very good acting, it was acting. Some girls barely even try to make it convincing, but some sell it really well.
That said, I've had a few girls where it was quite clear it was real. As others have said. During DATY the secretions change, there are involuntary muscle responses, etc. There's one fave of mine (no longer at Oase) who acted very similar to an ex of mine- when she orgasmed she clenched up and kicked out with her legs. Made for a very, uh, interesting experience at times. It was clear hers were real not just from her spasms, but when she would get up she was literally dripping on the floor.
As for frequency. Ever girl? Eh, no. Multiple times per girl? Meh, maybe once, but rare if ever. Do I appreciate that the WL's who fake it convincingly? Absolutely. Do I always believe it? No.