It's useful to do a bit of risk analysis.
Purely electrically, discharging a 120 V capacitor through your finger is very unlikely to immediately kill you, but likely to give you a burn mark. Discharging it through the body carries a greater risk, but realistically, you will probably be startled but fine. Keep in mind that universal power supplies will often boost the voltage to about 300 V in its first stage, to have a smooth, well defined DC voltage to work with. The power stored in the capacitor will relatively quickly be dissipated through the DC/DC converter and the capacitor's voltage will stabilize at the DC/DC converter cutoff voltage, and will then very slowly discharge through whatever leakage path are in place. The purely electrical risk analysis will be the same.
The bigger risk in this case is actually your reaction during the shock. Startled as you are, you may do any number of more or less dangerous movements. Maybe you will jump backwards and hit your head. Maybe you will charge your arm toward the exposed 230 V mains wire you have lying around the bench. Maybe you will pull out the oscilloscope from the shelf and have it fall over your head. You get the idea.
You can practically treat the capacitor, on its own, as being non-lethal. The more pertinent issue is to develop a careful more attitude to safety than thinking "this setup is safe, so I don't need to worry." What if you didn't turn off the power switch this time? What if you left that temporary test wire in place and forgot about it? What if you're making an assumption about the circuit that is not true, or the circuit is broken and doesn't work as expected - perhaps being the reason why you're debugging it in the first place.
You have everything to gain and nothing to lose from embracing a paranoid attitude about electrical safety. Always measure twice, every time. Never assume that a circuit is safe or discharged. Remember, accidents happen when you don't expect them.