I have very limited experience having had only a few subs and aiming to keep the current one forever.

For me the "test" was always more psychological than physical. I would simply bring in the subject of my taste for domination and then see if she would willingly submit. The irony about being a dominant is that it will only work as long as the submissive wants it to. So who has the real power in the end?

If after a first experience she is already willing to wear some kind of restraint and is willing to go further sexually than we already did without involving domination then I consider that she is a sub. When that did'nt work the relationship usually stayed without any bdsm and/or ended soon after.