Possibly stupid filament question ...  

RE: Possibly stupid filament question ...

A couple of years ago I had a loose coil of black filament and I was not sure if it was PLA or ABS.

I dipped the end in acetone and yes, it started to attack it, so it was obviously ABS and not PLA.

As for telling PLA from PETg, I know that often, but not always, PETg has more of a sheen than PLA, but I've seen PLA filaments that are very shiny on the surface and PETg filaments that are more matte.

The quickest way that I can think of to tell PLA and PETg apart would be to maybe heat the nozzle of the printer up to 180-190C or so, touch the filament to the hot nozzle, and see if it readily melts.  As a sanity check you could compare it to a known sample of PLA.

RE:

There are a few more tips but a I agree with JSW.  

Also:

1. Smell: ABS smells terrible and PLA is kinda sweet.  PETG has next to no smell.  
2. Brittle:  PLA is more brittle and easier to break than the other two when bent.  
3. Sheen: as per JSW
4. Acetone test:  ABS readily melts.  Some PLA will partially melt.  PETG will not melt.
5.  Melting point: As JSW indicated above, but I have found this difficult to interpret at times.  

I have often had to use 3-5 of these to figure some out.  I even decided to just trash a few rolls.   Not enough left to waster my time.  

RE:

Well, I tried it with NPR and it didn't dissolve it. Now gotta figger out if it's PLA or PETG. When I get home this afternoon I'll play with it more. Nice color and I have a print I want to do with it.

Oh, very shiny, might be PETG.