The best way to know if it's metal is to open up the hole just enough to slide a magnetic grabber in there; or a borescope. Magnets only stick to iron/steel, so it won't stick to electrical cables. They now make very cheap borescopes that plug into smartphones.
If you can touch it with certainty, you can make two voltmeter measurements (settings on volts AC ranged 240+ volts, do not set to amps unless you want to die).
- Mystery object to mains power "NEUTRAL"
- Mystery object to mains power "HOT"
Those will be the two major slots on your receptacles. If there's a third slot that's an oddball, that'll be ground which you will not use for this test.
Know your mains voltage (120V in North America, 230V in Europe).
If both numbers are a small fraction of mains voltage, then the object is electrically isolated from mains voltage and isn't an electrical wire... or you failed to touch it sufficiently - always a risk.
If either number is mains voltage, it is an electrical wire, or it's grounded (e.g. if all your water pipes are metal).
If the "mystery to NEUTRAL" number is approximately your mains voltage, you definitely hit a "hot" wire.
Hitting a neutral or ground wire is also serious. Current flows in loops; all the current comes back on the neutral wire. if you damaged it, it'll get hot there and potentially start a fire.