Fingers and eyes. There's a very good guide to what on look for on the Bank of England website. You don't have to be enormously discreet - you're going to put the money somewhere out of sight - and it doesn't take long to feel the paper, watermark and ink, look at the printing, shiny bits, watermark etc.
Pens / UV lights etc will catch some fakes, but certainly not all. Anyone trying to pass a pile of fakes will probably have made sure they pass those tests, so don't rely on them.