Normally there would be an indicator light somewhere, showing the gear pump is running. So, if there is a leak, one should spot this happening. The gear pump also drawn a lot of current which ought to be evident on things like the ammeter.
I had such a leak, internal to the gear emergency release valve. This valve releases the pressure within the system, allowing the gear to drop under its own weight (plus some gas springs for the nose wheel). This valve contains three tiny seals. Normally these are never replaced ( no need to) but on my last G-reg Annual a "new interpretation of EASA regs" caused them to be replaced, involving a great deal of labour and damaging them in the process, and they lasted only a few minutes into the flight. I released the gear and landed with it down; no big deal.