Fit a screw in the end and snap it on the collet nut of your router. Give it a spin and see which way your router is leaning. Usually you would want to shim the router to get it pretty close, for most people 1mm or less off with this tool is absolutely fine, smaller bits can get away with 2-3mm off, seriously.

Fits, RT0700 and RT0701C.

RT700C, RT0700CX3, RT0701C, RP0900K, 3621, 3621A