Feeding and Treats
Cavys have a very demanding body. Their diet MUST contain vitamin C, or they will die. Therefore, the pellets that you get for them to eat must be as high a quality as you can get.
Things to check for on your feed bag are; the mill date, or when it was made. If the food is over 90 days (3 months) old, the vitamin C has leaked out, and you basically have over-priced rabbit food.
The type of packageing, like if your food comes in a clear plastic bag, it is likely no good. If it has been exposed to sunlight then the vitamin C has gone from the feed. Thick paper bags are best for packageing.
Any tears or holes in the bag. If there are, then it could have been exposed to anything, including water. Once the feed has been exposed to water, it is only a short matter of time before it molds, and that is not good for your cavy, either. Mold can cause a variety of respiritory problems in your cavy, most of which require the help of a vet.

There are some treats to give your cavy that will help supplement vitamin C in the diet. Green peppers, parsley, oranges are high in vitamin C.
Carrots, apples-without seeds, cabbage, and lettuce are good for fiber, but don't contain vitamin C in the quantities needed. They are still okay to give as treats.
I also put a variety of dry toppers on my cavys' feed. Rolled oats(oatmeal), rolled barley, and a seed/nut mixture. This is made up of wild bird seed-no whole sunflower seeds- and a mix of un-shelled nuts, sunflower seeds, walnuts, almonds, and peanuts. I then put a tea-spoon of wheat germ oil on the top of all of this. The seed/nut mixture along with the wheat germ oil help with healthy skin and coat.

What to Look For in a Healthy Cavy
If you must buy from a pet store, please be careful of what you get. There are a lot of pet stores out there that don't know a whole lot about cavys.
Things to look for in a healthy cavy are...
1) Is it perky, or huddled in a corner like it will fall if it moves.
2) Is it bright eyed? Dull eyes mean a sick animal.
3) Is it fat? A skinny cavy is more suseptable to illness as it probably hasn't been fed correctly, and if it is too skinny, it will be dead within a day-almost guranteed
4) Does it have bald spots or sores from scratching? Then it could have lice, fur mites, or ring worm.
5) Is there any crusty white or yellow stuff around the eyes?vthis could be a sign of conjunctivitis or a cold.
6) Is it breathing ok? No crackleing, clicking, or labored breathing?
7) What is the condition of the cage it is in? Clean, Pine bedding, water bottle full, has food?
Just a few things to look for in a healthy cavy.

Bathing
I bathe my cavys once a month in a warm bath. I use a flea shampoo formulated for kittens so that it isn't so strong as to do harm to the cavys. This helps to keep lice under controll if they have been exposed, also.
I make sure that they are totally dry before putting them back into a freshly cleaned cage.