In hindsight after purchasing and experiencing so many, there are 2 things to look for:



1. Note the circle. Try to find as small a gap as possible. Otherwise, it seems that when you attach the arms in alt mode, this gap is widened. If you have little to no gap here, you should have a pretty solid alt mode.

2. Note the arrow. There was quite a variance in the level of burrs I saw, so look for a small one.

The above specimen is close to the best I've seen in both regards, although the picture might not communicate that. When in a shop comparing two, the differences should be obvious.