Which one should I use, the stool test or the urine test?

For monitoring a Gluten-Free Diet, both of them are equally effective. According to the Spanish Society of Celiac Disease, for effective control of a Gluten-Free Diet, 3 urine tests or 2 stool tests should be conducted within the same week every 3 or 6 months.

The urine test can detect gluten 2-15 hours after ingestion, while the stool test can detect it 1-6 days after ingestion.