How do you know if material is insufficient to win in xiangqi?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws for movement, checkmate and both repetition rulings: a side that forces a repetition by perpetual check or perpetual chase loses rather than drawing. The chase half has not been validated against the official casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it before publishing.
Direct answer
Compare what remains against the standard list. A lone cannon cannot force mate against a bare general, and nor can a soldier already on the last rank. A single chariot can, and so can a lone horse or a soldier short of the last rank. So can horse and cannon, two horses, and cannon with an advisor as its screen. Against an intact defence of two advisors and two elephants, the bar is considerably higher still.
Why a lone cannon is the special case
It needs a screen to capture or check anything at all, and a bare general in an empty palace supplies none, so a lone cannon cannot even give check. Add a single advisor and the picture changes completely, since cannon and advisor beats a bare general comfortably.
Checking before you simplify
Before trading down toward what looks like winning material, run the comparison: what will actually be left, and is it on the winning side of the list. Trading into a lone cannon against a bare general throws away a win with no resistance required from the opponent.
Related questions
Does insufficient material ever change with more soldiers on the board?
Yes, a soldier is often enough to tip an otherwise insufficient combination into a winning one.
Is a chariot against a full defence ever insufficient too?
Yes, effectively; two advisors and two elephants intact typically hold a lone chariot to a draw.
.