How do I find the rating on a main breaker box it there if not a label stating the amount?

What size main breaker is in it? There may be a label inside the panel, also.

This breaker box is for a house built in the 60s. There is not a label on the box or inside the panel...

I did not ask how old the home is, I asked what size main breaker is in the panel. Most likely that is the rating of the panel. I suspect you only have a 60, at most, 100 amp panel. If you had answered my very first question, we would know what size panel you have.