its just a simple blood test to find out. a rig pretty much is a stallion with one nut. he can still get mares pregnant, can become agressive and may fight with other horses. also, becoz the undecended testicle is at a high temperature all the time, he has a big chance of developing cancer. the vets will operate and remove the undecended nut

so definatley get the blood test, its not too expensive either