Maybe you just mistaken intact elerium for intact ufo power source? just seeing the power source core doesn't mean there is elerium in it, and YES it can be disginguished by looking at the power core: if at the base of the power source you see three dots, then there is not elerium in it. If you see those three dots covered by something that looks like a mindprobe/grenade, then there is 50 elerium in that core.
have you checked that?