How do I know if its the septic tank needing pumping?

Pumping a tank every year is MUCH too often except in the most extreme situations. Most tanks should be pumped out every three years or so, but depending on various factors you might want to do this more or less often. Since you don’t know when your tank was last serviced, you should have it pumped out right away. But if you're the only person who lives in the house, you can probably wait quite a while before you pump the tank next time. Talk to the septic folks when they pump the tank--they'll recommend a pumping schedule based on useage and tank size.

The timing you choose for pumping depends on how big the tank is, how much you use it, and how much solid waste it receives. If you have a big family or entertain a lot, opt for a shorter schedule between pumpings. Likewise if you use a garbage disposer—that puts a lot of solids into the tank. But if you have a small family, travel a lot, and compost all your kitchen scraps, you can probably go longer between pumpings. In any case, however, err on the side of more often rather than less. Pumping a little early won’t cost much, but leaving the job until the tank overflows will ruin your whole day.

If you don't know where your tank is, here's how to find out:

http://www.thisoldhouse.com/toh/asktoh/question/0,...

Septics should be pumped at least once a year.

You could also call roto-rooter perhaps you have roots invading the plumbing

Cost depends on your area, if in the U.S. about $80-$100