<p>So I have some old filament that I originally got for a 3D pen. The problem is it's unlabeled and I haven't been able to find anything that might help me distinguish whether it's PLA or ABS. The bag it all came in says that wherever this filament came from only makes PLA and ABS so it's got to be on of those two.</p>
<p>I have a roll of PLA in my 3D printer right now, but I can't tell if it's the same as the filament I have for the 3D pen. It's been a while since I've used the 3D pen, but I do remember whenever you used it, it would produce a very very bad smell. I've also noticed that the filament seems to be more flexible that the PLA in my machine. This makes me think it could be ABS, because the PLA smells far better than what I remember the 3D pen smelling like, and it's more flexible.</p>
<p>I also don't really want to do any heat tests or anything on the filament, so if the smell and flexibility is enough to determine which filament it is, could anyone tell me?</p>

Mick's suggestion is a good one. PLA may shed some color in acetone, but ABS will dissolve completely in a suitable amount of time. If you have dark filament, you can test by flexing the filament until it breaks. ABS will sometimes/often/usually fatigue with a white break line, while PLA does not exhibit this tendency as much.
PLA has a somewhat sweet smell, which may be the corn sugars burning off, while ABS has a much more chemical-like odor.
Not doing heat testing does limit your options.