A photo with something for scale might help.
The capacitance you can get by measuring them.
The voltage you could estimate based on the approximate date on manufacture, the side, and the capacitance.
It's possible that after measuring the capacitance of a few, some part of the part number will match. They could be 48uF, and some other par is the code voltage (not very useful) or they might be 1uF and 48V.
If they're really old you'll have to re-form them. This involves charging them with a series resistor and a meter across reading the voltage, and a meter in series measuring the current. If they need reforming, the first time you charge them they will have quite high leakage, but this will reduce on subsequent charges. This generally exhibits itself as faster and faster charge times.
Re-forming should be done with the capacitor in an acrylic box or something similar that allows you to see any venting but also protects you from an explosion.
If you have a lot of these capacitors then you can charge one up to higher and higher voltages until it vents or explodes. Let's say it does that at 60V.
The next one of charge to 1/4 of this voltage and immediately discharge it (do this through a resistor which will charge it in about 5 seconds. For capacitance C in uF, time T, and voltage V, the resistance in ohms should be 1,000,000/C and the power rating about V*V/R.
Let's assume they're 1uF and the first one vented as 60V. The resistor should be 1MΩ, and the power 120*120/1000000 = 0.015W. That's tiny. Use a 1/4 W or 1/2 W or whatever you can find. For bigger capacitors the wattage will be higher.
Now connect a volt meter across the capacitor, and use the 1M resistor to discharge the capacitor if it has any significant charge on it. Charge it from a 15V source and check the voltage after 5 seconds. It should read 99.3% of your source voltage. Anything less may be due to leakage, or errors in your resistance, capacitance, or voltage measurements. Discharge the capacitor again (until the voltage is below 1% of your test voltage). Then charge it again and read the voltage at the 5 second point. Is it higher? If so leakage is reduced and the cap is re forming. Repeat this process a few times, or at least until you observe no further improvement.
Repeat this process by first doubling the voltage, and then by going in increments of about 20% of the voltage the cap vented at (in this case 10 volts would be fine). In this case the voltages would go 15,30,40, 50, 60, 70,... Measure the capacitance after each test. If it has risen, then your time to charge is now the new capacitance * 5 / the old capacitance ( so if it goes from 1 to 1.2uF, the test time increases from 5 to 6 seconds)
The leakage will increase with voltage and this will result in a reduction in the percentage of the voltage after 5 (or the corrected number of) seconds.
The test ends when the capacitor vents or explodes, the charge voltage no longer continues to rise, or you're charging to no more than 90% of the charge voltage.
For safety, I'd estimate the voltage rating at somewhere between 50% and 75% of the last test voltage.
If this is less than the voltage that the first cap failed at, you're probably good to go. If it is greater than that voltage then the capacitors will need reforming before use. You may get away with a slow charge to your rated village, or it may require several cycles at increasing voltages.
Sounds like a great way to while away an afternoon.
