You might consider buying a urinalysis type drug testing kit from amazon or the drug store and ask him to take the test. One would think that if it isn't him he'd be glad to take the test.
Alternatively (and at greater expense), if you wanted to go about it surreptitiously you could collect some of his hair and send it in for drug testing.
Wow
@negrogehas stated the most obvious solution here. It's obvious,but I sure as he'll didn't consider it.
If he won't come clean, threaten him with the drug test. Just like at the clinic, a refusal to take the drug test is tantamount to admission of guilt so... really you'll have your answer as soon as you even ask.
I think you know in your heart already. I'm a drug addict myself. I've been around addicts my whole life. When confronted with accusations, they're far more likely to double down and only admit guilt when caught red-handed, as it were, like "I have video evidence of you taking that shit"
I would recommend approaching the situation with compassion and understanding. It's your husband so I'm sure you're not looking for revenge or prosecution. Tell him you'll do any and every thing needed to support him, but from that moment on, dishonesty will not be tolerated.
You guys can get through this. Hopefully Hydrocodone is the only thing he is using. A sidebar, I'm hoping he knows not to take all of those pills without performing a Cold Water Extraction to remove the Acetominophen, which is safe in recommended dosages but quite harmful in larger dosages.