Good job with the coasters! The silver is heavier than you may think. I've profited even paying up to $5 each, depending on size. You can tell if they are sterling by looking thru the glass for the rust or green tarnish that disqualifies them. It's easier than hunting down the faint "sterling" mark, but seeing the mark is reassuring.
There's also the fingernail test: if you can get a fingernail between the metal and glass and actually pull it away from the glass, leaving a faint mark, you know it is silver. I'm a guy though, this is not something you should try if you care about your nails.