Compression artifacts are the visible errors introduced by lossy encoding. They have recognisable forms, and which form you are seeing tells you what to change.
Blocking (macroblocking). Square patches of flat colour, most obvious in motion and on gradients. The encoder ran out of bits and could not afford detail within each block. Cause: bitrate too low, or CRF too high.
Mosquito noise. Shimmering fringes around hard edges, especially text and graphics, that flicker between frames. Cause: high-frequency detail being quantised inconsistently frame to frame. Common on screen recordings pushed too hard.
Smearing and ghosting. Detail dragging behind movement. Cause: the encoder relying on motion prediction that does not fit the actual motion, typically at low bitrate with fast or chaotic movement.
Banding. Stepped bands across what should be a smooth gradient. Often a bit-depth problem as much as a bitrate one, and treated separately.
Loss of fine texture. Skin, hair, grass and fabric going waxy. The subtlest and the most common in mildly over-compressed footage — nothing looks broken, it just looks slightly synthetic.
The frames to inspect are the ones that stress the encoder: fast motion, hard cuts, dark scenes, gradients, faces, and any on-screen text. If those are clean, the rest of the video almost certainly is too. A quality metric like VMAF can tell you something has gone wrong; looking at these frames tells you what.