As usual, "It Depends" on what you have available, how much accuracy you need, on the angle itself, and on your lathe
I prefer my vernier protractor as well, but for shallow tapers like Morse, it becomes impractical to use, as things get in the way. Then the simplest solution is to just use a known-good Morse taper and a dial indicator to set the angle. For this, the indicator must be mounted as close to horizontal center line as possible - a good idea here is to use a flat foot on the DI; makes it much more accurate. Marv has an article on that somewhere; I can't lay my hands on it right now.
Kevin's method works well too - once again, the DI needs to be on center line, and you need to be fairly accurate on the length you travel with the slide to set it. On smaller lathes this can become a problem - there's not much travel on the top slide - especially for shallow tapers (close to parallel to the workpiece).
If you have precision triangles, those can also be very useful in setting the angle, especially if you can confirm if the side of your top slide is parallel to the top slide's dovetail. And even if there's an error there, it's easily compensated for. If you don't have precision triangles, it's very easy to make up your own that's usually plenty good enough for hobby use - it just involves a blank piece of material with three fairly accurately drilled holes and a milling machine - and some math.
For the "new age" machinists, some food for thought: A laser pointer can be used to set an angle pretty accurately; and I have my own ideas around that. There's some "off the wall" thinking required though
- oh, and some more maths...
There's many more ways to get the job done - and no definitive "right" or "wrong". At the risk of putting both my feet in my mouth and falling flat on my @rse: One thing I've learned about hobby engineering is that often times "close enough" is "plenty good enough". It's nice to work down to the absolute precision one can obtain, but there's no use in chasing absolute precision to the point where one gets to the point of "I can't do xxx because my machines/measuring equipment/skills aren't accurate enough" - then you'll never get anything done. Just go ahead and try the best with what you have - you might be pleasantly surprised at the results
Kind regards, Arnold