It's called a hygrometer and you should be able to get one for around $6. They need to be calibrated though since they're usually not right.
1/2 cup salt
1/4 cup water--mix in small glass bowl or coffee cup
Place bowl of salt/water mix and hygrometer (beside bowl, not in) in a large ziploc bag and seal.
Leave for 12 hours and read hygrometer.
Hygrometer should (but won't) read 75
If your hygrometer is adjustable then adjust it to 75. If not just make a note of how far it's off and add/subtract that number when you use it.
Mine said 71 so I add 4 to whatever it says when I'm using it in the bator.