How do I get my check engine light to come on again?

Months ago my check engine light started flashing. My dad recently disconnected the battery which caused the check engine light to shut off. Vocational rehabilitation offered to pay for repairs so long as i vet 3 diagnostics from 3 different establishments but I can’t do that unless the check engine light is on. My mom said to just drive it until the check engine light comes on again but wouldn’t that be dangerous seeing as there is obviously something wrong with the car since the check engine light was flashing?