Question

how can i determine which one is trying to destroy my garden. They eat roots and make mounds .  They make a hole away from the mound.  I have kind of kept them from much damage by flooding their holes.

Answer
You can certainly visit http://icwdm.org it has lots of information on identifying damage.

Short answer is
pocket gophers dig large mounds and eat roots. Pocket gophers don't leave holes open. they always close them. Voles and other rodents will leave them open.

Voles eat roots too but do not create mounds.

It is possible you have both.

Flooding could work some. But I suspect it makes you feel better more than being effective. Details on control methods that will work is on the site.