We could look at the person's eyes. Are they watery or bloodshot? Is the white of the eye pink? Is it bloodshot? Are the pupils dilated in bright sunshine? Are they constricted in darkness? Is the person fumbling with his documents? What is his speech like? Is the person answering questions in what we consider to be proper fashion?
There are a thousand different scenarios. We look at everything when we stop a vehicle. We check for, obviously, safety as one of them, but we also check to see if the driver is fit. Based on whatever indications you have, if you can tie it together and come to the conclusion that the person may not be fit to drive, that's when you would ask him to participate in a sobriety test.