Well, you have to get to know the project doing the airdrop more. You have to make some kind of a due diligence. Basically, it is the same process as getting to know an ICO whether it is legit or not. Because an airdropped coin and a coin offered in an ICO are both new coins which are being introduced by a certain project. So what you will do to know whether the coin is valuable or not is to carefully examine the project. You have to scrutinize the very important details. You have to know the team who put up such project.