well when you go to get that pitch gauge buy a swashplate leveler too. the one for the trex 450 will work as the main shaft diameters are the same. other then that you're going to have to eyeball it. you can try to use some system of bubble levels but it's too difficult I think. the other thing you can try is using a small T-square if you have one, line it up against the shaft.

I'm not a big fan of eyeballing anything, these are precision engineered and anything that is "off" will affect the flight with vibrations and poor handling.