if you have an INFINITE number of red marbles, what is the probability that one of them is NOT red?

The above comment is correct there is no way to verify that they are all red all you can really know is that you have an inexhaustible supply of marbles and everyone that you have checked so far is red from which you inductively reason that they are all red. This is a theory not a certainty and the first blue marble would disprove it.

If you have an infinite number of red marbles, then the probability of NOT picking a red marble is 0%

To play the Devil's advocate, if you have an infinite number of marbles, how can you ever be sure that all of them are red?

the probability that one of the RED marbles is not RED? should be 0%! (unless someone paints it or something)

there is no chance of any of the red marbles not being red, theoretically.

0. You said they were red, that implies all of them.

o%

one could have some dirt on it so idk