Joined: Jan 2010
Posts: 9,779
Likes: 1,748
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
Usually with derailers, if they're not straight enough, it's visible, as long as the hanger is known straight.
It's pretty easy, if the cogs are clean, to sight down the pullies along one of the bigger sprockets.
I check first looking down, with one pulley above the other, then rotate the cage to horizontal and sight everything down again.
A problem derailer always looks a little off, but sometimes it's just the cage plate and the shifting actually works fine in all the gears, a bent cage plate having relatively little effect on indexing.