Start with the floor. Walk every inch. It should feel solid and even under your feet with no bounce, no flex, no soft spots. That tells you a lot about the lumber quality and how the joists are spaced.
Next, check the doors. Open and close every one. They should swing smoothly and latch without forcing. Doors that stick or won't close tight are a sign of a frame that's already shifting.
Look at where the walls meet the roof. Tight joints and clean lines mean careful construction. Gaps, daylight, or uneven seams mean corners were cut. Check the hardware too. Hinges, latches, and handles should feel substantial, not flimsy.
Finally, look underneath if you can. The skids and floor joists tell you what the builder really thinks about quality, because that's the part most people never see.
