Well you can't be sure if there is an asic for a coin or not. They operate in the background and only start to sell asics when they get unprofitable or there might be some chance ofchange in algo in the future.
For example ZCash asic has been used nearly a year before they got announced to public. It just makes sense to operate like this. Earnings from owning andoperating asics is way higher then from just selling them ..
So asic proof is difficult because there is a high chance that every algo can be implemented into an asic. Maybe it is already and you just don't know about it!