Well, to do it properly, you need to check in two places.
First, you want to check the overflow bottle. It's a semi-clear bottle usually located near the bottom of the bike, often just behind the engine. You want to be sure the fluid in there is between the min and max levels.
Second, you need to check the amount of fluid in the radiator.
With the engine cold and the bike on the side-stand, pop the radiator cap off. It is usually located in front of the fuel tank on the right side of the bike. The coolant should be to the very top.
If those both check out, you're good to go.
Of course, if you don't know the last time it was changed, not is a great time to change it. Use something silicate and phosphate free, like Prestone Extended drain. If you use silicate coolant, it will eat your water pump.
Currently Running Stable:
2010 Honda NT700V
1986 Shadow 700
1986 Honda Trail 110 (Postie Bike)
1987 Honda Rebel 450
1973 Honda Mini Trail 50