Water bath. Will have to be dried throughly
afterwards (2 big fans, left overnight), and you need to check that there aren't water-sensitive components first, but that's how I inspect PSUs.
Other general advice is wearing rubber boots and use a single arm (and doing this with a friend). This will prevent the current from going through your arms or discharge through your feet (passing through the heart in the process), and the friend will be there to rescue you in case of electrocution (knowing Basic Life Support
procedures would be optimal for both, any first aid manual has them, if you saw at least one episode of Baywatch
you should know them as well). They call it "the buddy system
". Never do something dangerous alone. Remember the movies.
If you get a discharge with this setup it will be painful and you may lose sensitivity/movement of the finger/hand for a few minutes. Better than dying anyway.
I don't use rubber gloves as I drop enough stuff already, with gloves I'm less precise than a 90-year old with parkinson.