Most salebarns will check a bull if you buy one there. The local barn here charges $35. Probably similiar price if you take in a bull you bought else where to your local vet. Not sure what the electro-stimulation procedure runs, probablya little a bit more. Unless you know the owner/seller personally and have seen calves from particular bull, you really should get him tested. No reason to waste 3-4 months just to save $35.
While this will tell you his fertility level, it wont tell you anything about his libido. Therefore the pen full of cows would come in handy.