How to detected invisible ink?
Basically, the deck should be replaced regularly, which is the best protection against marked cards at all. Are the cards marked raggedly, helps so called ripples of cards, similar to the flipbook to detect irregularities.
Casino employees are trained the pattern on the backs of the cards mostly very familiar, so even the slightest changes immediately stand out.
Be aware that it is not particularly difficult to mark a deck, you need to actually just a red pen. A very good preparation of the subject can be found in Card Sharks.