Use a clear plastic drinking straw as a dipstick. Hold your finger over the end when you pull it out to keep the fluid in the straw. Itís not extremely accurate, but it will give you a good idea of where the level is.
I think we need a few more details. Did you clean it out then replace the fluid you took out of it? Is the Radiator topped off and free of all air bubbles? Does it have fluid and youíre just not able to see it?
Make sure there is fluid in the Radiator as well as up to the line of your Overflow reservoir. It should just go up to the Max line when cold. Also you will want to get the reading from the reservoir while the bike is standing up straight or on its Center stand depending on the year. If its on its side you may not be able to see the fluid. Older bikes tend to be harder to read cause of the aging on the reservoir makes it hard to see threw at times. Not sure how a straw is going to get threw to the Reservoir though it has a bent neck just below the passenger peg. Would be hard to get much of anything threw there to read it.
Unless you did a complete flush you should only add fluid threw the Reservoir. Try and make sure itís the same color fluid as well. Phosphate-free is recommended.
Use a bright flash light from underneath or from the other side. Check the level while the bike is on the kick stand too.
Underneath works best if you're alone. PITA. Get down on your knees and reach way under, shining the flashlight into the bottom of the overflow reservoir. I use a small LED flashlight -- 8 or 9 LEDs. Always have to move it around some to get the angle just right.