Five minutes with your eyes on the welds before you hook up. Work around the trailer, mark each spot, and you'll get a summary at the bottom — including what to photograph if something doesn't look right.
Before you start
This is a quick visual check of the welded structure — it's not a certified inspection, and it doesn't cover brakes, bearings, tires, or lights (check those too). Cracks hide under paint: look for hairline lines in the paint, rust streaks bleeding from seams, and flaking along welds.