How to determine unknown capacitor values

What kind of capacitors are they? How old? And what are you going to use
them for? If they are electrolytic, I would throw them away because electrolytic
will dry up and leakage will increase with age. Also, I wouldn't use old cap (NOS
or otherwise) in high voltage application like tube amp because you can't
measure max working voltage of a cap easily.

In general, I would avoid using old caps because they bring more trouble than
they are worth.

There is a common believe in the audiophile circle that everything NOS
means good. It is not. All electronic components deteriorate over time, some
faster than the others. Unless you have sophisticated measuring equipment at
hand, you wouldn't be able to tell the good from the bad.

You need what's called an "LCR meter", which looks like a multimeter but measures inductance, capacitance, and resistance. MCM has one, and Parts Express has a digital multimeter that can also measure inductance and capacitance so it can function as an LCR meter.