It would really depend on their sizes. I would assume that if newly purchased they would be about 1-1½" in length with minimal girth? If so, both could still be nueter or if slightly larger male. If either is closer to 2", then you may have at least one female. If both where female, you would know quite quickly. They would try to kill each other.
Pairing will happen almost all of the time, especially if they are still immature but really depends on the species of clown. It's only once they have matured to far (1½-2") and placed together that it is a risky venture. Bonding behavior will look like short lived spats, squabbling without injury, chasing, one or both will shake violently and so on. You won't know if they are a mating pair until they lay eggs.
Cheers
Steve