Place the blade on a thin pole through the center hole of the blade. If it wants to fall to one side - that side is heavier. You will need to grind of some more steel on the heavy side to balance the blade. You don't need to grind more of the angle down - just grind a little steel off the back side or a corner of the blade. If you don't feel comfortable with grinding your own blades, check around for a service in your area that will do it for a few bucks.
Reel grinders require their own special equipment for sharpening.
You can buy your own or hire a service to sharpen and grind for you. Reel mowers work just like a scissors, the reel spins against a bedknife and the grass blades that are caught in between are severed. The blade and the bedknife should never actually touch - they are designed to come close enough together to cut a piece of paper, but not produce any friction from rubbing. If you cannot turn your reels by hand because they are rubbing against the bedknife - you have them adjusted incorrectly. The same phenomenon applies here - the reel touching the bedknif as it spins creates friction, which creates heat.