Well, it's tricky. Normally a field sobriety test would be a good general indicator but THC doesn't cause horizontal gaze nystagmus so it's more subjective. Plus cops lie, so...yeah.

To my knowledge there's no "weed breathalyzer" and even if there were THC concentrations aren't necessarily indicative of level of impairment.

Haven't had the chance to look at the legal codes for states that have legal cannabis usage but I'd be curious to see what they've got on the books for this.