Examine the material of your fossil closely. Obvious fake materials include plastic or rubber. Less obvious fakes may be resin imprints of the original fossil. Look to see if the coloring of the fossil is uniform or varied. A fake fossil is more likely to have very different colors throughout the fossil. Similarly, small holes in the fossil may indicate a fake, as air bubbles up during the resin hardening process.
Use a sharp object such as a needle to scratch the surface of your fossil. Do not push to hard, or you may damage the fossil. Check to see if the color comes off when you scratch the surface. A number of vendors create fake fossils by dying the fossil to disguise its age. This dye should come off when you scratch the surface. (Ref 2)
Shine a portable UV light on your fossil. If the fossil was made with a resin, it will reflect the light. If it is authentic, it should react to the light in the same way as surrounding materials.
Check for damage. It is extremely unlikely that you will find a completely intact and undamaged fossil. If your fossil looks too good to be true, it probably is.
Saw your fossil in half if you are sure that it is a fake. Since you will destroy your fossil, you should only do this as a last resort. Look for a hollow space underneath the fossil -- this is a sure indication the fossil is a fake.