How do I know if my projector is 4:3 or 16:9?

You can tell by the resolution of your projector although most projectors will throw both 4:3 and 16:9
4:3 ratio resolutions to look for would be, VGA, SVGA, XGA, SXGA
16:9 ratios would be 480P, 720i/p, 1080i/p