If one had a large tub of old LEGO brick, with some pieces going back to the 1950s, but all mixed together, are there any distinguishing features or appearance to the 1950s cellulose acetate bricks compared to the newer ABS plastic? Do the plastics look different somehow? Or would you just have to look for other features like lack of studs, flat bottoms on 1x1 round pieces, etc?
LEGO switched from cellulose acetate (CA) plastic to ABS plastic in 1963. They continued to distribute CA pieces in sets until their existing supply was exhausted, so during the transition beginning in 1963 both types of pieces were sold in sets. In North America, LEGO pieces were manufactured beginning in 1961 under license by Samsonite, which continued to distribute CA pieces mixed in sets until about 1970.
Distinguishing CA from ABS pieces based on intrinsic criteria is something of an art.
The first thing to look for is warping. CA plastic is much more prone to warp than ABS, which is perhaps the main reason the switch was made. If you find an old piece that is warped, odds are it is CA. You can check for warping by placing a piece on a flat surface and checking for any "wobble." Another good method is to build a simple brick wall with the pieces and look for gaps or poor connections.
Generally CA plastic is much shinier than ABS plastic when it is in good condition. CA pieces will lose the shine with wear, so this is not always possible to see.
There are also some tell-tale clues in the colors.
As 62Bricks has said, this can be an art.
Based on my experience (in the UK):