WHAT YOU SHOULD KEEP AN EYE ON DURING THE PURCHASE
Have a look on the cage where the animal is kept in, if it is very dirty and stinks, you should not buy any animal in that pet shop.
Most pet shops are already separating males and females. If this is not the case, it can be that you buy an animal which is already pregnant.
Watch the animals for quite a while and then try to tend to a more active animal, which runs around and "talks" to it's colleges, then for one which sits motionless in a corner.
If you have chosen an animal, let the shop assistant give it to you and then try to look for the following:
- The fur should be bright and heavy.
- A shaggy fur with bald specks point on a disease.
- The eyes should be bright and moist.
- The nose should not be moist or encrusted.
- The ears should be clean.
- A disease of the ears can also be diagnosed, when the animal tends to let it's head hang to one side, or if it scratches a lot on the ears.
- The anus should be clean an not smeared with muck.
- The underside of the paws has to be smooth and without scabs.
- The weight and the age are also important points. Unfortunately those are only very hard to get to ascertain in a pet shop.
- You should now, that the babies are still nursed up to the 4th week.
Therefore, I think, it stands to reason, that younger Animals are not fully developed yet and still would need their mother.
Most certainly you will not get an answer on this question in the average pet shop, because the emplyees simply do not know or care for that.
I have seen deliveries to pet shops, where the Breeder has been selling two weeks old babies. That's what I really would not call responsible minded!
But from the weight you can at least guess how old the animal is.
(More see Development of the youngsters)
If your guinea pig fulfils all these criteria, you will not have to worry a
lot.
For sure this is not a guaranty, not even a veterinarian can give you such.

Underneath you can see a perfect example for a good pet shop.
The pen is clean and spacious, the genders are separated from each other and one can watch the animals before buying them.

Of course there more possibilities to get a guinea pig. Here are some:
Directly from the Breeder
In you local animal shelter
Through adds in your local newspapers
Family and friends