How could I determine if one rock is denser than another rock?

Answer 1

You must be undergoing an identity crisis. You've posted six chemistry questions, using three different names.

Please keep the same user name for Jiskha posts.

Answer 2

Weigh each rock and record the weight. Drop each rock (I assume these are small and not boulders) into a graduated cylinder partially filled with water. Read the two levels of water in the graduated cylinder (before dropping in the rock and after dropping in the rock) and subtract them. This gives the volume in cc. Then mass/volume = density.
If you do have boulders, and you are allowed to deface them, chisel a piece from each and subject each to the above test.