I am (and have been for years and years) prescribed 8mg of Xanax daily and have taken 20mg and higher single doses while eating my legally prescribed Roxicodone (oxycodone without acetomenaphen/paracetamol) as well as street bought opioid pain pills, shooting heroin (and I sold heroin for a good while, so my dope was good since I made the product myself and used the same dope I sold, and I generally did 1 and 1/2 to 2 grams per day for years and years via the IV ROA) and drinking liquor all day and into the night). I did this for a long time and was always fine (I'm a real good guy for HR huh? Actually, I don't recommend doing this combo, even at lower doses than mine, as even a fraction of that can kill you easily. Be safe, know your limits, and use common sense. You can always take more, once you ingest a substance or cocktail of substances it's gonna have combined and increased poly drug effects and you are basically at the mercy of your tolerance). I haven't touched heroin in years, but I still take my prescribed Roxicodone and Xanax combo every day for chronic pain and panic disorder, and unless you need these medications, they can be a real pain in the ass to keep up with once you become physically dependent on them. Since the state in which I now live is very nazi-esque when it comes to opioids, I'm grossly under medicated on the roxy when you factor in my pain levels and more than 20 years of opioid and benzo tolerance levels (Xanax dose is spot on). This is why I normally run outta roxy early every month and have several hellish days of increased pain, coupled with physical withdrawls that suck ass, until roxy refill day. Point being, even with a legitimate prescription, physical dependence can be a pain in the ass.
As to your question, I'm sure you're aware that Xanax has a very distinctive taste, as do opioids. While I'm just offering a bit of advice, not saying it should be your number one test in determining adulterants in pills, I would taste a piece a pill (assuming you're buying more than one) and chew it without swallowing. Pretend it's a wine tasting and swish the chewed piece around your mouth, especially all the areas of your tongue, while checking for the subtleties and nuances of the Xanax taste (plus any benzo taste, as they all have their own subtle nuances of flavor but all taste like benzos, IME anyway. It may be a mix of Xanax and Valium or some other benzo or thienodiazapine, for example) while also tasting for that bitter opioid taste (same as with benzos, all have their own flavor, but all have that opioid taste. Again, I say this from my own personal experiences. Your palate may vary). That's about the safest and best way I can think of for a quick check of what you already think will be some cocktail of stuff put into a pill press. As I said, it's not a rock solid method that should be your only way of testing it, but if the friend has the pills in hand and you've got the cash in hand and it's time for making a deal, I think it's a pretty good way to do it. Something to consider. Oh yeah, while counterfeits are often crumbly and the real pills are usually kinda "shiny" when light hits them right, I have personally had many legitimate, real pills that were old and were crumbly, so crumbliness isn't always a 100% indicator of a counterfeit.