The best way to gauge how they are doing is regular, weekly, weighing.
Sometimes a pig may look like they don't eat much, but as long as they are steadily gaining weight (for young pigs), or holding a reasonable weight (mature adults) you will know they are ok.
For example we weigh weekly, and noticed one of our guinea pigs has lots quite a lot of weight.
On closer examination it turned out she had snapped both of her top front teeth off, and was struggling to eat.
If we didn't weigh regularly she would have lost a lot more weight before we noticed.