If you are only building a handful of units for non-critical applications you might just use the "smoke test." If you are designing for a longer production run it might pay to varify with the manufacturer that the cap is suitable for the application.
When in doubt use a conservative design which in the case of caps mean you a significantly higher voltage rating than you calculate or measure. When you have things working you run an extended "smoke test" than then shut down and give the components the "digital temperature test." If you say "ouch" when you touch a component, it may be running a little hot but not necessarily and that's why you read the spec sheets.
You will often see "hints" in the suggested applications section of the data sheets or catalog description such as "suitable for switching power supplies" or "recommended for capacitor input filters."
You have to pay attention to "frequency" effects as you go to RF as at some point the inductance of the leads will resonate with the capacitance. All this is why when you want a power supply designed you go to a guy who has done it before, etc.
Just for fun, I dug up a "control unit" for our old deep well pump that failed. This is just a motor starter with a current relay. Interesting enough, this cap isn't marked as a starting cap so I suspect that many replacemnet caps are only sold in the "run" version. The label includes: 88-108UF 330 VAC 50/60Hz +65C. Clearly this cap is designed to not fail pre-maturely when it runs a little warm. "Doing the math", 65C is 65*(9/5)+329F. That's up in the "ouch" range but just barely. Above 140F is considered dangerous for domestic hot water.