There are a number of methods for identification of fake Rudrakshas like:
Sometimes rare Rudraksha beads like GauriShankar Rudraksha or Trijuti are made by artificially joining two or three Rudraksha with the help of glue, etc. In case of doubt, such Rudraksha bead should be placed in boiling water for 20-30 minutes. A sharp discoloration will take place at the joint in case of artificial Rudraksha.
Water test can be conducted as well. The bead is placed in a glass of water. If it floats it is rejected. However, this test is not very reliable because : An unripe but genuine Rudraksha will also float in water. Fake Rudraksha bead made of wood and impregnated with lead will also sink in water. Rare Rudraksha beads like One Mukhi are sometimes made by tampering with higher Mukhi beads. This Rudraksha will also sink.
-Cross Section View Test
The most reliable test is to cut the Rudraksha bead. One will find the same number of compartments as the number of lines. However, the drawback is that the bead is destroyed.
The Mukhis of a natural Rudraksha look like closed lips with deep natural partitions. Sometimes Rudraksha are faked by sticking thorny parts of one Rudraksha onto another one. This difference could be caught by a 10x lens.
However, none of these tests are fool proof. The best way to buy a Rudraksha is to buy it from a trusted authorized supplier.