Im wondering if any of the ladies doing BBFS are wearing female condoms, which may be hard to detect

BBFS is using nada. A female condom is easy to detect if you look. It’s not the most attractive thing out there & it does protect against STD’s, so it’s not BBFS.  If your a visual person & can see you will notice the female condom it’s not the same as BBFS. I have heard providers say it’s like being raw, but I don’t have a penis, so I can not compare.

Female condoms cover the pussy lips, it protect better against herpes but last I have checked they cost $3.00 a pop. They are nice but not worth the price, they can still get tricky.

Firstly, if the FC wasn't so cost-prohibitive,  
I'd regularly integrate them into my repertoire.

I usually carry FC2's with me for the guys who have big issues with traditional male condoms.  Yes they cost more then male condoms.  And I doubt anyone could get away with pretending is was BBFS.  Like already mentioned you can see them, as a portion of the unit remains outside the woman.  
 They are almost impossible to find in a store in OC or southern CA for that matter.  It is just easier and more cost effective to find a good deal on Ebay and buy them in bulk.  
 It is the closest thing to BBFS that I can/will/do offer.  
Some guys really like them, or maybe I should say prefer them to wearing a male condom.  
They do need a good amount of lube in them to be good and comfortable and have that BB feeling.  
 I like to use warmed  lube when I can.
Also, it takes a little bit to get them inserted and properly placed.  But I have never had a guy say he could feel the inside ring during sex.  
 I have also heard that they are popular in the gay mens community and used for anal.  Which I think is smart of them to do.  Since I do not offer anal, I have never tried this.
In my opinion,  it is the closest thing to BBFS and still practice safe sex!

Doesnt seem that expensive to me for the protection and pleasure it gives.