Materials fail in a small number of recognisable ways.
It looks like plastic
Almost always roughness. A uniform mid-roughness value with no variation is the plastic look. Real surfaces vary — smoother where worn, rougher where dusty, different on edges than on flats.
Second most likely: metallic set to zero on something that should be metal.
It looks flat
- No normal map, or one too subtle to read.
- Ambient occlusion missing, so nothing grounds the detail.
- Base colour with no variation. Flat colour reads as flat.
- Lighting, not material. A single light with no bounce flattens everything regardless of the surface.
It looks dirty or muddy
Ambient occlusion applied too strongly, or baked into base colour as well as supplied as a map — applying it twice. AO belongs in its own channel, subtly.
The colours are wrong after import
Colour space. Base colour and emissive are sRGB; everything else is linear. Getting it backwards makes roughness maps too dark or too light, and normal maps subtly incorrect everywhere.
Highlights are in the wrong place
Normal map channel convention. OpenGL and DirectX disagree about the green channel's direction, and a map baked for one and used in the other has its lighting inverted vertically. The fix is flipping green — and the tell is that surface detail looks like it is lit from below.
Seams are visible
UV padding too small, or normals not matching across the seam. Both are fixed before texturing, not after.
Metal looks like painted grey
Metallic is not set, or the base colour is too dark. Metal's base colour is its reflection tint and should be bright — dark metal usually means a wrong value rather than a dark material.
Generated textures specifically
They commonly arrive as base colour only, with lighting baked in from the reference image. Both problems compound: flat because there is no roughness or normal, and wrong under new lighting because the old lighting is painted on.