You can look at your records. You know how much you took out from which lenders at what percentage and for graduate or undergrad. You will have to see how those numbers work together to determine the under portion.
For example:
Loan A is undergrad for $20,0000 but paid off to $6,000
Loan B is grad for $40,000 and paid off to $18,000.
Loan A $6,000 + Loan B $18,000 = $24,000 debt owed total
Loan B 18,000/ total $24000 = 75% grad debt