How to check driveline angle
The proper way to set the driveline angles is to measure the angle of the rear end yoke and the angle of the transmission output shaft using a magnetic base protractor to establish the base angle. Once this is known, then the driveline angles can be adjusted by shimming the transmission mount, or using tapered shims if you have leaf springs, or adjusting the links if one is running a 4-bar suspension with adjustable links.
Most builders will measure the angles during the mock-up stage in order to get the all the mounts fabricated to the proper dimension. The better builders will leave themselves a means to adjust things during final assembly in order to tune the chassis and correct any mis-measurements.
 How about using a level?
Since a proper magnetic base protractor is available for little money, using a level to measure driveline angle isn't usually a reasonable solution.
Below is a picture of a magnetic base protractor. You can pick them up at Sears for around $20.