You can only see the front few of a piggies teeth and usually if problems occur it is further back in the pre-molars as Wiebke said. Mostly these are not easy to see without special equipment and some vets can find it difficult to see them even with the correct equipment as piggies are good at keeping food in their mouths.
Weight loss, dribbling and dropping food are good indicators that there is a dental problem. The incisors (front teeth) that you can see should be a good healthy colour with no cracks, not loose, the lower and upper sets should meet... Dental problems of the back teeth can be hereditary, diet related or without known cause. Incisor problems often occur if a piggy has a fall, is dropped or otherwise manages to bash their face/teeth (I have had piggies with both!)
If a good specialist vet is doing a dental assessment they may also suggest an x-ray to check the roots for overgrowth, abcesses etc. Often the visible problems are the symptom of a problem further down in the tooth or jaw.
The TEAS dental sanctuary does an amazing job and the experience of Furryfriends who runs it is phenomenal. Have a read of some of the cases they have dealt with to see how much good dental care can contribute to a piggies health and survival.
At home the best thing you can do to promote goo dental health is feed a balanced diet with unlimited hay to allow the back tteth to wear naturally as the piggy eats and if they have a wooden toy/house or something they can keep their incisors in trim too.