Is the fabric in my formal shirt a dobby or jacquard? How do I tell them apart?

Both dobby and jacquard create self-woven (woven-in) patterns — but they differ in complexity, loom type, and the scale of pattern they can produce:

Quick identification test: If the woven pattern is small and geometric (tiny dots, micro-squares) — it is dobby. If the pattern has curves, florals, or large-scale motifs — it is jacquard. You cannot create curved patterns on a dobby loom.