The difference is hard to tell, they are both part of an 11 species complex of closely related species. A. clarkii is very wide spred and has several color variations depending on geographic range, dominance and even reproduction. Also because of cost and range, allardi isn't often found in the american pet trade and when it is it is 3 or 4 times the cost of clarkii. There is the possability of mis labeled fish being sold at the wrong price, but I would guess it would be the other way around, a clarkii being called and sold as allardi, not the other way around. Allen and Fautin have a great book called Anemone fish and their host anemones, that details both sp and has some varifying marks, but some can only be told apart by collection location and things like # of scales and fin rays. Because of the variation among individual clarkii, if the one you were looking at was with a bunch of other clarkii clowns I would bet money it was a clarkii.