When it comes to dating knowing whether or not a person is available is one of the most important things on the checklist.

If you want to ask if someone is single you would use the word soltero for a male and soltera for a female:

Are you single?
¿Eres soltero/a?
¿EH-rehs sohl-TEH-roh/ah?

When asking if someone is single, you use a conjugation of ser or estar as the verb. Depending on the area where you are, you might find one used more than the other, but they are generally interchangeable in this context.

He/She is single.
Está soltero/a. (estar)
eh-STAH sohl-TEH-roh/ah.

He/She is single.
Es soltero/a. (ser)
ehs sohl-TEH-roh/ah.

Solterón (sohl-teh-ROHN) for males and solterona (sohl-teh-ROHN-ah) for females are a stronger way to say soltero/a but their meanings are quite different.

If a male is said to be a solterón this could mean that they are either a confirmed or long term bachelor. This isn't to say that they might never want to settle down, but chances are they are more of a player than someone interested in commitment.

On the other hand solterona tends to categorize a female being an "old maid" which is much more derogatory than the male version, so you wouldn't want to ask someone ¿Eres solterona? or you might be wearing a drink instead of sharing one.