Not everything old is worth saving, and deciding which antique fixtures are worth restoring is part of my job. I pass on more than I take, and that judgment happens before a fixture ever reaches the bench.
Is it still sound?
The first question is what's there. A fixture can look rough and still be sound, or look fine and be past saving, and telling which is which is the skill. Solid brass or bronze with a tired finish, I can bring back. A body bent past true, or pot metal that's crumbling, or a piece missing so much that it'd be more reproduction than original, that's usually a pass, or a parts donor at best.
Can it be saved honestly, and is it worth the hours?
The second question is whether it can be made safe and whole without becoming a fake. I can rewire almost anything. What I can't do honestly is rebuild a piece so heavily that it stops being the thing it was. The third question is plainer: is it worth the hours? Restoration is slow, hands-on work, and some pieces would take more time than they'll ever be worth. Sometimes I take a hard case anyway, because it's rare, or because it can be saved and nobody else would bother. But mostly I'm honest with myself about what a piece can become before I put the time in. So when something's here, it cleared that judgment: sound enough, saveable without faking, and worth the work.