Eyeballing will do, but if you wanna be anal like me I use a cable tie to level the swash on my precision machines (not my Genius, just my 400, 550, etc).
A swash leveller tool is an unnecessary expense.
The cable tie method works well, and it's been well documented so I wont elaborate here.
Just search youtube for "cable tie swash levelling" for future reference.
Here you go.. http://www.youtube.com/watch?gl=GB&h...&v=iUAluAxRi4Q