Identify cap type by dielectric loss?

[Circlotron]

I have a bag of 100nF 630V and 220nF 450V caps that I was told are polypropylene. Is there any way to verify this by measuring the dielectric loss or some other way?

[TimFox]

The two likely dielectrics in that package are polypropylene and polyester (Mylar).
At 1 kHz, polypropylene will typically have Q values of roughly 1000, but polyester will typically be between 100 and 200.
A simple LCR meter such as the DE-5000 will be more than adequate for this measurement, or an old-fashioned capacitance bridge.