Blood tests are pretty unreliable, small intestine biopsy (not colonoscopy) can give a false negative if they just happen to miss the patch with the flattened villi in the small intestine. If you're one of the lucky people who gets a rash from eating wheat they can biopsy the rash instead. If you have "geographical tongue" which is patches of flattened taste buds that is caused by celiac disease but can't be used as a test. They won't biopsy your tongue.

The only really reliable test is the genetic one.