Fentanyl?

Hi. I’ve been a recreational heroin user for a while. I live in eastern Europe, so I never really had to worry about fentanyl. I moved to Philly, usa for studies and I’m worried they sell heroin mixed with fentanyl here. I’ve heard that the difference between them is that if you take fentanyl on heroin tolerance, it will knock you out more quickly. So, is that true? I’m planning on taking very little at a time. I don’t wanna carry naloxone, I don’t wanna carry drugs either. I’m gonna take little amounts just in case it’s fentanyl, and if a little amount knocks me out, then it’s fentanyl. Will this work?