When not flying in combat, they would have their transponders on, allowing ATC to see them just like any other aircraft.  Modern ATC radars don't depend on actual skin contact, but the transmissions from a transponder to locate and id aircraft, although they do still have the skin contact ability.

Active radar reflectors or flying with the gear doors open

You hear them pretty regularly on the center radio frequencies out over Kansas, Nebraska, Missouri etc., especially at night.

The B-2's have the coolest callsigns, "Death", "Reaper", etc.  Of course all you hear is the civilian controller's transmission to them but you know it's military.  The controllers will point them out as traffic to look for if they are close enough.

Of course there's always some wise ass that responds by saying "the stealth must be working cause I can't see them...." <-----------LAME

Couple months ago, we had a pair of Raptors cross out flight path, 1,000 ft. above us.  That was pretty cool to see.