There are lots of new piggies arriving on SFA (and welcome to all of them)
While you are getting to know your new piggies (and they are getting to know you) start a weight log
In your diary or (if you are a computer buff like my DH I'm not techie you can start a spreadsheet)
Some good kitchen scales.
A bowl or plastic box to pop them in.
Weigh the bowl/box to subtract from the total
Try to weigh every week and monitor.
It gives a good idea of how your pig is progressing.
Check their overall bodies for lumps and bumps.
Eyes
Ears
Claws
Teeth
Bottoms
(Each time you cuddle them)
Guinea-Pigs are complex little mammals and if they do change bodily, the quicker you pick up , the better.
And you'll find all us guinea-slaves getting very excited about pooh - the texture,colour and quantity of our cavies pooh 
And pee 
I weighed our piglings today :
GP3 is 3lb ( one year old Rex boar)
GP4 is 1lb 14oz (ten month old smooth sow)
GP5 is 1lb 8oz (ten month old smooth sow)