Get a piece of cardboard/plastic etc and put a line on it about 1.5mm (roughly 1/16") from the edge. Stick it in the tread at various points. If the tread's not deeper than the line at any point, you need new tyres now.
At 17000 miles I'd be surprised if there's ANY tread left on your front, and there won't be much on the rear. If you look at your rear from above it'll probably be square instead of round.
Matt
