Do they have any wrap on them with numbers. If you can discharge them after fully charged, that will give you a exact capacity at a certain discharge current. I use a hobby charger but that may be a little more than you wanna spend to just find out the capacity of each cell. But a hobby charger will let you charge a pack probably quicker no matter if the cells are series or parallel. Just food for thought.
If you can find out the cells make and model it should be easy.
There are other ways to get a rough idea of capacity.