As hard as it is to believe, there are a handful of guys that have a slot to fly fighters the day they walk into flight school. The Guard trains some guys from scratch, they don't take just prior experience guys. When you get selected for a Guard spot, you are already slated to go to the unit sponsoring you and you will fly what they fly. So if you get picked up for a pilot slot with a fighter unit, you know from the day you start that you will probably fly fighters.
There are a few caveats to this, the biggest one is that you can still fail out like anyone else. And if you suck but graduate you might end up flying something else, a lot of state ANG's fly more than one plane. Finally, the unit might change aircraft and you either change with them or try and fly another unit, which is not easy. For example, the ANG unit in Georgia that was flying B-1's switched to E-8's, and a bunch of the old F-4 units switched to C-130's or KC-135's.
I learned a lot of this when I went through Nav school, my class had a lot of Guard guys. Every single one of them was prior service in the unit they were going to fly with or a similar Guard unit. They said the fighter slots were extremely hard to get, you had to know someone (Bush's Guard unit had Lloyd Bentsen's son and a few other sons of prominent Texans in it). And before anyone gets too excited, the AF is thinking about cutting the number of fighter units in the Reserve and Guard when the F-22 and JSF come on line, fewer fighters to go around. Though the Virginia ANG is going to train with the 1st Fighter Wing to fly the F-22.