is there a way they can test you if you smoked marijuana in the last 6 hours or so.?

my friend said that marijuana will never be legalized because if someone were to drive under the influence of marijuana, if marijuana were legal there would be no way to test if your *high* at that minuite. as for alchohal they can test your breath. so i am wondering is there anyway you could test if someone were under the influence of marijuana. i mean i know you can test to see if its in their system but if marijuana were legal it would be ok to have marijuana in your system but not to drive and be high. i know they could check your pupils but that isnt a accurate way of telling all the time. they would need a test that works all the time. is there a way of telling if you have smoked in the last 6 hours or so?
 
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