Simple.
1 -First make sure you have your reserve tank full.
2- Empty the tank and put in a liter or two of petrol in it.
3- Note down your odo meter reading.
4- Drive around until you motorcycle stops due to empty tank.
5- Note down reading and switch to reserve so that you can go and refeul your bike as normal.
6- The difference between first and second reading is you average/litre.
motor cycle have reserve tank so this can be done without much hassle. Do keep a notice of odo meter reading whenever u refill. then you can keep a track of your average change as well.