Originally Posted by a.noctilux
The best way is weighting the lens:
133g is "plastic" and 155g is "metal"
I have a "Lens made in Germany" version of the Summicron 35mm f/2 v4 (pre-asph) that has a silver chrome finish. The weight is a hefty 240 grammes. The serial number is 3612523. Beautifully finished, even prettier than the Summilux 35mm f/1.4 Steel Rim.
Boy, I wish that this one was made in black paint, but it isn't.