There's probably a testing kit for it, but I wouldn't take the risk if you're unsure about what you got. Fentanyl is strong, and in recent years there has been a slew of deaths resulting from fentanyl laced heroin taken by people who had no idea.
^exactly that with the addition of marquis reagent turns pinkish when used on a sample containing heroin so it definitely works for opiates but not sure about fentanyl. And just restating this because it can't be said enough, most people can't even see the amount of fentanyl it would take to kill them if IVed.
there is no way to tell. i dont understand why dealers would cut there shit with fent when they could just sell the fent as is. if you have even the slightest suspicion GET RID OF THE BAG that high is not worth your life.