How do you determine if Japanese Rifle is 6.6 0r 7.7 Caliber?

Determining the type of rifle would be the first way. If it's a Type 35, Type 38, or Type 44, it was originally chambered in the 6.5 cartridge. There were also some Chinese Type 13 Mauser rifles chambered in that cartridge.

The Type 99 was the 7.7 rifle, and resembles a Mauser somewhat. However, many of these were rechambered. The Nationalist Chinese rechambered them for the 7.92x57 cartridge during the Chinese Civil War, and the US rechambered many for the 7.62x63/.30-06 cartridge to provide South Korea with rifles during the Korean War. You should take it in to a gunsmith and have them measure out the chamber.