If you have an insulation tester, just measure the capacitor's insulation resistance at about working voltage. If it's above 1G then it should be fine, below 100M I would definitely discard it, in-between, it depends on the operating conditions. Sometimes you can do this test in circuit, if you can isolate the capacitor by pulling the fuse, or switching off the power switch if it has one.
I have tested a number of these Rifa capacitors. My conclusion was that their failure is quite predictable, the mechanism being thermal runaway due to the insulation resistance having a high negative temperature coefficient.
A new Rifa from Kemet is specified to have an insulation resistance above 12G. I have some unused 10 year old 100nF Rifas with an insulation resistance of around 1 to 2G at 25 degrees Celsius, measured at 250 volts. Old cracked ones that I have tested come in at around 10M or less. These are the ones that are now primed to self destruct.
Regardless of age, the insulation resistance of all those that I have tested approximately halves, for each 3.5 to 4 degree Celsius rise in temperature. I don't know if the new ones from Kemet exhibit the same characteristic, or if my 10 year old unused Rifas ever met the specification of the Kemet ones.
If the ambient temperature inside the equipment is very high, above perhaps 60 degrees Celsius, even 1G of room temperature insulation resistance may not be enough to prevent the capacitor reaching the tipping point for thermal runaway.