I have double doors that I would like to repaint. I'm pretty certain that they are not wood, but how can I tell if they are fiberglass or steel or whatever else they could be? If someone could give some tips I think I can figure out how to proceed from there. Thank you.

How old are they? And, why are they needing to be repainted? I just finished repairing and repainting some doors that were damaged some.

Find a magnit , if it stick... metal
Fiberglass... I just doubt it, they hold paint really well... my gut says you don't have this type door
Wood or metal is what I'm guessing you have, and if there exterior id bet you have metal

Regardless of what it is the prep work is all the same, scrap or sand all loose paint, prime with oil primer or laytex primer, the oil is more durable, then go with a laytex paint making sure its exterior, some will say use oil, which is fine it just take all day to dry, laytex will dry in a few hours

Use a magnet check for metal, or get a ladder and open the door and scrape the paint off the top of the door. This is a good inconspicuous area to test for wood or fiberglass.

You are going to paint so scatch by the hinges with a pocket knife. Metal will be bright silver, fiberglass and PVC will look whiteish and woodwil look like... wood. Clough and Glavine gave good advice. When you get ready for the top coat I paint all trim with a good brush but use a fine roller for the flat surfaces. Add Flotrol to the paint if you want a very smooth surface.

use a magnet check for metal, or get a ladder and open the door and scrape the paint off of the top of the door. this is a good inconspicuous area to test for wood or fiberglass.

Well this could possibly not work, for many Fiber Glass doors are wood framed with a Fiberglass sheeting over the wood frame.