You would have to test his/her sample with a validity test strip to see that the creatinine and specific gravity are within satisfactory limits. These are essentially the same colorimetric tests that are used at the lab when they run the validity tests on the sample (which had become SOP in recent years, making sample "color" the least aspect in determining an overdiluted sample).
These kits are inexpensive and are found online by doing a Google search for "drug adulteration test kits"

Without the validity tests, you would have to chance it by taking his/her word that s/he had not consumed an abundant amount of fluids prior to donating the urine sample to you (iffy, iffy).

Do you have access to a sample from a different clean person?