From the way you described his appearance and behavior, I would assume that he's been taking some sort of depressant(s) - most likely marijuana, possibly in conjunction with alcohol or pills. Nothing you described is indicative of him using cocaine or any other stimulant, though, so I guess that's a plus.
As for ways to tell for certain - if he's using marijuana, you would most likely be able to smell it on his person. That only applies if he has been smoking it, however, as there are other methods of using it that don't involve combustion. If he's on pills, I can't be of much help there, as I was able to hide a 10-year pill addiction from every single person I know - it's easy to do if you're careful.
How much of the day is he in your presence? Does he leave for a while and then return home looking/behaving the way you described?
I forgot to say that those are meetings for loved ones of users...past or present.
Oh...and I would highly recommend that you find a local meeting of Nar-Anon or Al-anon and attend at least once. You can tell your story safely and without judgement. They can give you great advice. A quick web search should help you find a meeting near you.
It's not possible for us to know. It does sound suspicious and it's probably best to trust your intuition. He was already telling you lies before so you are always going to have doubts which is never a good place to begin a life together. When he supposedly "quit" before you would have seen him go through withdrawals. Did he? It takes more than a couple of days. Most likely he would have been anxious, restless, and seem to have a bad flu for at least a week...probably more. At any rate I would ask him to attend premarital or couples counseling and make sure the prior drug use and selling were discussed. Please don't marry him or even move in together until your fears are dealt with and you are convinced he is clean. Would he take a drug test for you if you asked? If he has nothing to hide he'll agree. If he gets upset and complains that you don't trust him..... well that is a pretty good clear hint that he's not clean. You can but tests at the pharmacy. Read all the directions before and watch every step of the test. Drug addicts and amazing cheats and liars from long practice.