Buying replacement knobs - 1/4" or 6mm?

I have a Belles preamp that has some broken knobs. Is there any easy way to figure out if I need 1/4" or 6mm replacement knobs? Is it more likely that my knob shaft is one or the other? I assumed that with a set screw, either would work, but now am not sure.

Thanks.

Take one of the knobs off and try to insert the dull end of a 1/4" drill bit in it. If it slides on fairly easily you may be able to use either size. If it doesn't go in, it's probably 6mm. The difference between 6mm and 1/4" is pretty small, about .014" so worst case is the set screw has to reach a little farther.

Thanks - that was a perfect recommendation. Really appreciate it.

1/4 inch happens to be 6.3 mm. Worst case, since it has a setscrew- buy the smaller and then if needed, drill it out.