There must be vacancy, or the unit must be growing. You can find out about openings by checking out each unit's website. Do a Yahoo search for the unit you want. Also, check out baseops.net. Great site. (sorry for the advertisement Doug, just thought it would help)
About finding info on a slot, Ive found out through my attempts that the best way is to call some of the higher up's in the unit or call recruiting and ask for the phone number of the person in charge of hiring and just call them and inquire about the current availability of pilot slots and when the next boards are, UTA's, etc.....
An active duty C-17 pilot friend of mine told me that pilot slots in the reserves or guard are extreemely competative to the extent that you pretty much have to know someone in that unit and do some networking. Much like in the civilian world.
For the most part the ang promotes someone already in the unit 98% of the time. Probably depending on the unit of course, that's just what i've been told. ie if there is another officer career available for example a navigator I would seriously think about taking it, then trying to work up that way. I'm not sure about the reserves, I believe quite a few of their pilots come from experienced active duty pilots. Hope this helps a little just from my experience.