Crack pics:
Red bike is an Alien ACX, seat-tube detail. Note the crack behind the weld. I've seen them much worse than this.
Orange bike is actually a Nishiki FS-3, but it was built using the same tubing, same welding process, virtually the same geometry, although the head tube has thinner walls than the ACX head tube along most of its length (except the top and bottom where you can see it's thicker).
The all-aluminum Aliens tend to crack in the same places as the ACXs and FSs. There are probably other areas prone to cracking, but in my limited experience, those are the 2 most common areas cracks can occur.
Maybe you can show us some pics after you spiff it up?