You cannot check capacity of an unknown battery, Without charging or discharging it, which in your case is not feesable.
You can check the voltages though: choose the pack with the highest voltage. And over time, as you check the voltages of packs, you'll see that (FOR EXAMPLE), 8.2v is fully charged, or less than 8 is discharged. You'll notice general resting voltages that can indicate charged, or discharged, but you cannot precisely predict Mah.
Hope i helped, John.