If I had the urge to drug test someone I would take that as a sign that I am not comfortable having them in my home. When I see my sister once a year during the holidays I notice her pupils and I am always conscious of where my purse is. That's terrible - but it's a pretty good indicator that I am not comfortable enough with her to invite her into my home.
The boundaries are to protect ME - not to make her prove anything. So my comfort is paramount when it comes to my boundaries. I guess what I'm trying to say is that it doesn't even matter if she is actually high at that time or not, the mere preoccupation with such things is detrimental to my recovery, so until I am comfortable she won't be welcome in my house. I think this is why so often it makes sense to extend boundaries to "early recovery" as well as "active addition" when it comes to this scenario. And if she is in early recovery, I certainly don't want to be harassing her with all of my issues. My need to know her status at all times is MY issue, not hers.