depends on the resin. testing for ammonia is a good precaution, but, without knowing the composition of the resin/plastic/paint or stain, you cannot be sure that it will not leach a harmful chemical out. bad plastics leach out more than just ammonia...
odds are its safe, however, you cant know for sure untill you try. my suggestion is to buy a feeder fish,comet, or gold fish. drop the statue in the water, and if the goldfish lives for a couple weeks you are prolly safe. if the goldfish dies, you got your answer. it may be a cruel test, but if you care about saving money on decor so much, the death of a 50 cent fish shouldnt bother you.