Sure thing. \o/ Hakkai wears power limiters which make him seem human, but I think it'd be fine if Crowley sensed something. Although in Saiyuki's canon, the youkai/demon thing is actually considered to be another race aside from human, not demon in the same way Crowley is. SORT OF LIKE ELVES. Only youkai are mortal and, er, aren't always as pretty.

*BABBLES*