Obviously this should be disclosed well in advance of any discussion so your SB is out of bounds. Whether you should ask every POT is a lot more delicate.. I would think they might be offended if you brought it up out of the blue. Unless this becomes a pattern I would avoid..you might if you have doubts mention something like you're eager to connect with a 'real woman' but nothing obvious.

This is the kind of thing they talk about on the reddit forum.. you would probably get a couple hundred responses with all points of view.

I think is is pretty rare, so the risk is low.  So I personally wouldn't bring it up to every pot SB.  It's actually never happened to me out of maybe 50 or so they I've at least met (and more so of those who never got that far.)

But I don't think that the natural born females are aware of it.  Most trans SBs identify themselves as such right away.  After all, they don't want to be disappointed or dissed by a guy who finds out and is pissed.  I've never been in conversation with a SB who only then revealed her trans status.  I would be annoyed if that happened.  Occasionally I have seen a profile that says seeking trans friendly and I have asked politely about that, such as, are you trans or a natural born female?  I don't think you need to ask every SB you contact and converse with.  

I've set my search filter to exclude any profile with "Seeking Trans Friendly" and "Seeking Transgender" tags. This reduces my search result count by around 2-3% in my area.

 
I've never been in your situation, but I have been viewed, favorited and messaged by POT's who either show trans status in their profile, or mention it in the message. It's a hard pass for me (no pun intended).  But even though I have seen an increasing number of trans women on the site, it's not at the level where I need to proactively ask.    

 
As I said in the past, I'd really like SA to add a required, (now "non-binary" is the new format I suppose) gender-specific  field to the profile which can be a used as a hard filter in search.  If a trans doesn't use one of the Trans tags, you might see that status in the profile name, tag line, or perhaps buried in the profile text.  Or you might not know until you're texting, like the OP, or perhaps not until you are toweling off after BCD.  

 
There's room for Trans in the Sugar Bowl - every one deserves to play in this space. But like all things in a sugar arrangement, transparency and mutual consent are the foundations needed for success.  

 
Life is good

 
The Cat

No there was no trans tag. Not my cup of tea either and was really ticked that it took so long for her to say so.
 I do have my searches set for no trans stuff, but when I go to the home page I click on women I like and so maybe those aren't filtered.