The two can be distinguished by moving animals to see if non-kin are treated the same as kin. If so, then reciprocity is probably the cause; if not, then kin selection is probably responsible. The altruism is never ultimately costly--it is a question of apparent immediate cost vs. long term benefit to one's genes.
Work Step by Step
The key here is to see that the difference can be tested by switching animals from interacting with kin to interacting with non-kin to differentiate between kin-selection and reciprocity.