I think those that know the addict is using opiates and see the addict frequently are able to tell.
Those that don't have the contact or knowledge - can't tell.
My ex was an opiate addict for years - but I didn't know and couldn't tell.
My AB is different - same opiates - and I could tell within 5 seconds of walking into the house if he used that day. I could tell in the emails he sent to my work if he used. I could just tell. I could actually see AB in the mornings, and I would know if he was going to use that day.
Now bring your personal addict to me, and I might not be able to tell.
You can ask and see what her response is - might not be what you want to hear or it might be a lie.
But actions do speak louder.