The race is on!
Flash capacitors are usually big round things that run on 330V. Putting a screwdriver (holding it by the acrylic handle) to the poles will give you a nice spark if there's still juice in it.
Flash capacitor size will vary, as will their operating voltage (up to 5kV for some xenon stuff I think). The voltage of course is not the issue, but how much current they can discharge (and how fast). The larger, generally the more dangerous (more current capability) and as said, they will make a nasty spark, vaporize metal, and cause both flash-burns and heart-stopping jolts if you are not careful.
Capacitor current capability has increased over the years, to the point where a disposable camera flash charge circuit exceeds that of an older camera/flash easily. A small finger sized cap can now deliver current in excess of an old D-cell sized monster from a few years ago. Thus, it is imperative to remove all batteries and ensure flash circuits are discharged before working!
(Though there is nothing quite like the CRACK of a dead short across a large current capable capacitor - flash or otherwise!)
