If the V ways are worn,especially the front one,there will be a little vertical "cliff" at the top of the way. You can catch it with your finger nail. The cliff will be most significant of the side of the V that faces the chuck. This happens because on the lathe's carriage,where the V's are machined(on the carriage) there are keyway like grooves cut into the female V's so that the tops of the V's on the lathe bed do not touch anything on their tops. Therefore,as the bed wears,the beds' V's will begin to go up into the keyways on the carriage.
The more severe the wear is,the more pronounced these "cliffs" will be. Ultimately,when you get the lathe back together,turn a metal bar several inches long,supported at the outboard end by the tailstock. Make the last cuts very light. Measure up and down this metal bar. If the lathe is worn a lot,the metal test bar will be shaped like a wooden barrel: It will be larger in the center than it is at both ends. You can make a lot of short parts just fine,but will have to resort to carefully filing longer bars while they rotate,to make them acceptably parallel. The only real cure is to have the bed re ground,or re scraped.