There are things you can look for.
Realistic shadows, that actually follow the moving object.
Lights, which you can place and match to the shadows.
Figures that actually have a lot of mesh to them, as opposed to your basic boxy figure covered with a 2D painting that is shaded to look like 3D. This usually only works from one angle.
Skies with clouds that aren't all the same and move realistically while the figures move.
Figures that aren't all the same--a lot of times you'll realize after a while that all the models are actually a very small number of variants, like "the fat guy" or "the broad shouldered hero" or the "bosomy chick."
Any sort of moving object that glows or gives off its own light. Look to see if the light is cast realistically on other objects as the thing moves.
Look off in the far distance and see if there's a point where the world just sort of stops, or if it seems to go on forever. This is often really obvious in long dungeon hallways.
A lot of games either take place all on one height, or just move up and down in a very limited height range.
Any or all of this combined at once on the screen is probably the largest test of your graphics quality. A lot of computers can handle one big ugly monster, but when you get multiple monsters, the game either slows down or things start to look not completely right.