As you can see a water level is the most popular low price solution. A few years ago I showed guy that was putting new siding on his house how to use a water level. He thought I was a genius for coming up with such a good method but I had to confess that it's an old trick probably a hundred years or more old. In old books I have seen them made by putting glass tubes in the end of an old rubber hose. The only real advantage of using a laser or transit is ease of use A pole barn or most residential building don't need any more accuracy than you can get from a water level
A good technique to actually do the leveling is to get an old bed spring and drag it back and forth.
Scp