Using a line level it pretty easy once get the hang of it. Just pound two stakes into the ground, one on each end of the area you need to take measurements from, and stretch some mason line from one post to the other. Mason line works best and is pretty cheap at your local hardware store. Tie the line to one post, and pull it tight around the other before tying it off. Make sure you do a pretty good job of pounding in the stakes, as there will be some tension on the line, and you might need to keep the stakes around for while until you complete your job.
Once you have the stakes in and the line in place, put your line level - also available at your local hardware store for a couple of bucks - and hang it on the line towards the center. Then move the line up or down one or both of the posts until the level gives you a level reading. Now you can measure down off of the level line to see what kind of grade your working with and go from there.