Open the free STL in the slicer you already have on your computer. Rotate the model around and look closely at the preview — if you see any red highlights, open holes, or surfaces that look like single paper-thin lines, those are missing walls or broken geometry. Right in the middle of checking those areas, I learned that truly reliable 3d printed things come from models that show a solid, closed mesh without any gaps when you scroll through the layer view. Here is a fact you can use right now: most free game character models were designed for video game rendering, not for printing, so they often have zero wall thickness or surfaces that face only one direction. Just run a quick slice preview without changing any settings — if the software shows empty spaces where the walls should be or layers that appear and disappear randomly, that file will fail on your printer. No extra programs needed, just the slicer and your own eyes.