how do you determine what gears to run at different tracks?

Best is Ask the Track Owner / Promoter if he does not know he'll find out, he wants your attendance.

ask other racers then pay attention on the grid, if they tell ya say 15-66,67 look at there's if indeed rear is 66 67 chain will be closer flush with gear guard, if there not running a gear guard you can see the number, then you just need to verify there tell n the truth about the front. Of course ask the faster guy's.

Also visit other tracks when you can even if you do not wish to race, then ask pay attention and keep notes.

Or make sure you know exact track size and just ask on here, there's a lot of guy's with lot's of experience that will get ya started out.

Good Luck !!
Make sure you have a tach. And as you do it for a while, you can get to where you just need to look at a track and you can usually guess at a good starting point.
Asking to try and find out what front driver most are on. Thats the key. Then we usually start at 64-66 rear for the first practice and make adjustments according to the tach. If you dont have a tach then your going to be out in left field. Every motor, every chassis is different so what works for one person may not be the same for another. Its all in getting your motor to running in its sweet spot.
thanks guys,appreciate the feedback..never thought about looking at chainguard...i'm learning karters are like fisherman,they not gonna tell ya where to catch them..
yes sir,i I have..would it be fair to say clutch at approximately 50 percent total engine max rpm"s..

that would be fair. its a starting point. flathead 7000/ 3500 animal 7500 / 3750 both maybe 1-300 low
clone 6200/ 3100 same likely 1-200 low
clone,animal sr.champ.thanks for info..

If it's a true 1/4 mile, I see your SC and assume Maxxis tire and a day race where track surface gets pretty good I'd start the animal on 18-65 and the Clone on 19-64, there starting points pay attention to your tach I'd want my rear gears for animal feature to be between 63 and 66 and for clone to be 60 to 62.

Good Luck !!
What track would be a big help on helping you with the correct gear. A 1/4 mile track can be much different bite wise which equates to speed.
And wouldn’t the turn radius, and/or, the length of the straights make for changes in stagger and gearing? Maybe weight balance?
I’m always wondering about the distance between the straights. This would give you a good reading on the turn radius and the length of the straights. Nobody seems to care, so perhaps I’m just anal about the subject.
Still, giving out stagger numbers, without this information, seems like guessing at best.
Keep your front true - that clutch ferrule isnt made of steel and will lose a lot of whip if you dont keep it clean and oil after every race.