STL files are based on coordinates rather than individual units of measurement as they're supposed to be completely unit agnostic in order to allow them to be evenly scaled on any platform, and to not be bogged down with unit translations.
Typically, the units are set by whoever makes the model, and by whatever the default of the software that they use is. In most cases, creators use a one-to-one scale.
So, the answer to your question is that the conventional scale is "100%". Though probably isn't the answer that you're looking for.
If you print it at 100% it will be whatever size the creator intended it to be. For example, if you were printing a Warhammer miniature and it has a base attached to it, and you print it at 100% then that base will be 25mm wide, and the figure will be the exact height that the creator intended it to be. Whatever that height is.
For the purpose of your question, if the part "looks" like it was created in MM then it almost certainly was.
If you take that STL file and put it into your slicer, then use the default scale (100%), you can use the slicer to determine how its width or height or length in MM. Just set the scale of the model to whatever makes that dimension the same as the dimension you need it to be and the rest of the model will be scaled accordingly.
Your biggest problem might actually be the material that you use, as some will shrink after printing, or shrink unevenly.