Simple: let it grow. If it turns into purple zombies and eats the town (or otherwise shows unexpected gene expression), you failed. A variation on this is to a

Simple: let it grow. If it turns into purple zombies and eats the town (or otherwise shows unexpected gene expression), you failed. A variation on this is to also have it do an easy to check change, like various forms of fluorescence, then check for the easy to check ones and assume that if that one happened, the important one happened too.