There's never a point to the "meet me at the park and we'll fight it out" kind of stuff. It's also extremely dangerous. Too many people will bring their friends . . . or a pair of guys named Smith & Wessen . . . Heckler & Koch . . . you get the idea. Don't ever agree to this.
The only smart way to do this is to invite them to a boxing ring, or an open sparring session. And I'm sorry to say you're unlikely to meet anybody with enough maturity to accept those terms. The kind of people with something to prove always want to do it on their terms.
If you get time to talk, say something like "Look, you're totally cool with me. I don't want to fight you, but if you hit me I'm going to hit you back." Mean it and project it with your whole self. Say it matter of factly, and with no malice. It might work, depending on how smart the other guy is.
Lots of times, especially when somebody just loses their cool for a second, attacks will come without followup. Somebody will throw a punch, and then start regretting it as soon as they do it. The best thing to do here is just to block it, dodge it, or just take the punch and laugh. Hell, say thank you afterwards and just go about your day. No point to these kind of fights either.
Conversely, if you do get into a fight, or if you lose your cool and start one, don't think for a instant that there's a way out. Once you make an attack, don't stop until the fight is over or you have a clear means of escape - and watch out for the other guy's friends! The key here is to survive, and I'm not joking. An untrained kid is totally capable of lethel force.
I don't know how you'll ever tell the difference. The best I can say is good luck, and keep a good heart.
This kinda stuff never goes away. Keep your head up. Try to rise above it. Hopefully it all works out.