While you can spot gross geometry problems by eye with a test signal, there is just no way to precisely correct minor errors that way. Back when CRT RPTVs were big, people used to make and sell patterns to put over your screen for geometry purposes. Your best bet is probably a measuring tape, string and some tape. Measure out where the strings need to be for proper alignment, stretch the string taut, tape the ends to the frame of your TV and then use the strings to get your geometry lined up. This method won't be as perfect as using a pattern, but it'll help you correct the major problems hopefully.