Hi there, I assume you mean the transmission's angle in relation to the rear end as in pinion angles? I use an angle finder to do mine. You will need to know the angle of the transmission/engine at the tailshaft. You can use the flat machined part of the yoke for this usually. Then you will need the angle of the driveshaft itself, just put the angle finder on it at 90* and then you need to get the diff pinion angle. Once again use the flat machined part of the yoke. These together will tell you the angles your u joints are working at. Just a note that all of these need to be take at ride height. Different suspension types will want different settings and you will also get a lot of debate on the internet about this. As a general rule of thumb I like to see leaf spring cars around 3* down at the pinion and a 4 link about .5* to 1*.
Where people get messed up is that those degrees I am talking about are the angle in relation to the driveshaft. Not the ground. The relationship of your rear yoke to driveshaft is your pinion angle. I would suggest googling the subject as there are several good diagrams out there that can explain it much better than I can.
Best of luck.
