i think some dive shops will do tests for you, i think HPAC are your best bet, bit more expensive though. You may have to send it to them and they send it back (as far as i know)
You only have to test your bottle when its test date runs out, or if you damage it etc. Most bottles have a 3 year test on them, although some of the newer ones have a 5 year test.
Whatever date is printed on your bottle add 3 years and then thats when it runs out.