Most animals make their own vitamin C. Guinea pigs cannot, just like people. If it is missing from the diet for long, a guinea pig can become seriously ill, so the answer to "does my guinea pig get enough?" is worth checking rather than guessing.
The good news is that it is easy to cover with ordinary food. A veterinary school guide puts the need at roughly 10–30 mg a day, and a welfare guide for schools gives 10 mg per kg of body weight. Both fit the same simple plan: good pellets, plenty of vitamin C-rich vegetables, and fresh food every day.
Tap the veg you offer in a typical day. The tool adds up a rough vitamin C estimate for one guinea pig, using food-table values for raw vegetables and a typical handful. It is a guide, not a lab test: vitamin C drops as veg wilts, and every pig eats a bit differently.
Pellets and hay are the base. A few vitamin C-rich vegetables every single day cover the rest.
Bell peppers (any colour) are the easy winner: lots of vitamin C, and most guinea pigs love them. Parsley, kale and broccoli are also rich, but go gently. Welfare groups list kale and broccoli as foods for a few times a week rather than every day, and broccoli and cabbage can cause gas in big amounts. Lettuce, cucumber and celery are fine and hydrating, but they give very little vitamin C, so they should not be your only greens.
Sources differ on a few details. The RSPCA lists citrus as something to avoid, and the PDSA says fruit only in very small amounts, so treat fruit as a rare nibble, not a vitamin C plan.
This guide is about everyday feeding, not diagnosing. Contact a vet if your guinea pig eats less, loses weight, stops passing droppings, seems stiff or reluctant to move, or if you think it has eaten something poisonous. A guinea pig that stops eating can become seriously ill quickly. If your pig has a bladder or urinary condition, ask the vet before changing calcium-rich foods like kale and parsley.