Since you do not have a tachometer on the dirt bike you will have to listen and feel where the bike pulls the hardest.
First find a long flat stretch of land. Start moving down the stretch and then open the throttle wide open. Listen and feel how the bike starts to pull. Where the bike is pulling the hardest, is your power band. If you leave the throttle opened long enough in any one gear, you will feel the revs rise and the bike will stop pulling. This is when you have ridden out of the power band. Now do the exercise again, this time when you feel the bike reach the point where it is pulling the hardest, you then shift to the next higher gear. This will keep you operating in the meat of the power band.
The way you keep the bike in the power band is to shift. You will be up-shifting and down-shifting constantly on the race track. This is where open class bikes and 4-strokes have an advantage. They can go into a corner in the wrong gear and "lug" out. A race 2-stroke (especially 125's) need to be kept in the power band to accelerate. The main part where people get out of the power band of a 2-stroke is cornering. As you are braking for the corner, you should also be downshifting at the same time. This ensures that as you slow your speed you will match your transmission and engine rpm to that slower speed, so that when you go to accelerate out of the corner you are in or right at the beginning of the power band.
Some guys will use the "fanning" clutch method. This is where you will quickly pull in the clutch while exiting a corner to kick the rev's up, due to you entering the corner in the wrong gear. When I ride and race, I try not to fan the clutch. I try to always be in the correct gear for that section of track, but keep it in your "bag of tricks" in case you need it.
Modern 2-strokes (anything made after 1990) are waaaaaay different to ride than the ones made in the 1970's and 1980's. The early 2-strokes used to have light switch power bands, especially the 125's. They would lug and then hit all at once and then literally drop off as fast as they hit. You would have a 400 to 600 rpm window of where the bike made peak power. Modern 2-strokes thanks to their "power valves" have increased the power band over a couple thousand rpm's. So the power band is much more mellower and distributed over a wider range. You won't get the BAM! YOU ARE THERE kind of response out of a newer 2-stroker. This is what made racing an open class 2-stroke so difficult, you had 50+ horses coming on all at once like a light switch. Those days are long gone and trust me, they are not missed.