Okay, this is being picky and my head is full of books at present (it won't be for much longer).
But this is the truth of it.
Someone, one day, will use the supply as their 'proving unit'. They will test the supply as 'live', they will isolate what they think is the supply, they will re-test on their 'dead' supply and during that time their voltage meter will have failed, be it batteries or mechanical failure. Then they will assume the supply is dead....and it isnt.
Stranger things have happened and its the law of averages that such an occurance will happen.
Im only playing devils advocate here. This is unlikely to happen, but this is why 'proving units' are the recommended way of showing your voltage meter is operating correctly, hence proving isolation of the circuit you are going to work on, and protecting your safety.
So, Mattmoo......your 'known supply' is free alright, unless the batteries run out just at the wrong time, then it could be pricey.
Unless your approved voltage indicator will work even without batteries. Just like this one
Martindale VT12 Voltage Indicator
i thought most did to be fair?