This is the easiest way to quickly tell a sub from a Dom without advertising that you may be interested in jumping the poor submissive and making her cry for hours.
Simply task them with holding your flogger. If she takes it and holds it like this:
|submissive holding a flogger|
Basically, any way where she holds it 'for you' to take back from her when you ask: she's a sub.
However, if you ask her to hold your flogger and she holds it like this:
|Domme holding a flogger|
and perhaps starts swinging it around to get a feel for it like this:
then she is definitely a Domme. Dom/Dommes will almost always swing a flogger that is new to them so they get a sense of how it feels and if they like it. How heavy is it? Is it balanced. Does it have a nice grip that won't slip out of your hand? Or perhaps just, "What would it feel like to flog someone with this?"
I find this method far easier than asking which is sort of the BDSM equivalent to asking, 'So, do you have a boyfriend?" and can lead to awkwardness all around. But 'Hold my flogger please" gives you all the information you need while being subtle.