That’s such a real struggle - I’ve definitely downloaded models that looked amazing on screen but turned into a mess of unsupported, paper-thin parts once I tried printing them. After wasting filament on a few “just for rendering” traps, I learned to check for solid, manifold geometry and look for mentions of tolerances or support settings. What really changed things for me was finding a marketplace where every file is tagged with its purpose; on https://www.gambody.com/  they separate display, cosplay, and printing-ready categories clearly, and they actually test the STLs before listing. Now I always scan the description for print tips - like suggested layer height or assembly pins - because that’s the fastest clue someone intended it for real plastic, not just a beautiful render.