I don't think you can know only by how you feel when you imagine yourself in each role, as going through with something IRL is different to imagining it. (like, just for instance, I can imagine skydiving and not feeling uncomfortable at all; but I bet I'd be scared as hell and probably freeze up if I actually tried to it!)
Also you don't get your partner's responses when you imagine, and how your partner responds will affect your enjoyment of either role.. You may be dominant with one person and submissive or switch with another.
It isn't about sex only, it's about general behaviour. Are you a leader or a follower? do you come up with the ideas to go out or you follow suit, do you initiate projects (outside the scope of your job as a teacher) or you wait for the other party to suggest them. Do you feel comfortable getting your point of view known if it's against the general consensus.
On the sex side, have you ever initiated action with someone? whether real or online, or waited for them to propose to you? Are you waiting for that special someone to start a relationship? What kind of fantasies do you have.
Do you feel shy at your looks (eventhough you might be pretty) or you couldn't care less?
And finally on a funny note, if you're pissed at the questions you're dominant, if you're interested for more you're sub. (It's a joke)
if you have to ask your submissive
You know based on how you feel when you imagine yourself in each of the roles.
If you get a happy *tingleacheneedythrob* between your legs when you imagine giving up control to Another, and letting them decide what happens between you and to you (within negotiated limits), you're submissive.
If that *tingleacheneedythrob* happens when you imagine yourself as the one in charge you take the lead and set the pace, and direct and control your partner's actions and even *******), you're Dominant.
And if that *tingleacheneedythrob* happen no matter what you think of, you're a Switch and are happy in either role.
If you are planning on entering the world of BDSM and feel you're likely a switch, I strongly recommend you start as a submissive first. Dominants who have experience with submission are better able to get into their sub's head because they know what responses certain toys and sayings will elicit -- they really get what it's like to submit.