After handling it for a while, you kind of know whether it's silver just from the look and feel. Silver has a certain je ne sais quois to it that other metals don't. It looks like silver, it has a specific ring when struck, and it is very thermally conductive.
There are folk remedies like mustard and bleach that can give you hints, but like others said it only tells you that it's silver on the surface. And testing can damage the patina on antiques. But.. if it's plated, any wear at all usually reveals the base metal underneath.
I've had the most success just identifying what it is I'm trying to test. If it's from any known silversmith, there's probably a picture of it on the web.
Also.. remember that table knives with steel blades can still be "sterling silver." Hardly anyone ever made a 100% silver table knife; the blade would be too easy to bend. I had a bidder leave a nasty remark on one of my auctions once because "I called it silver and it clearly says stainless in the photo."
