Polarity in this case refers to the correct connection of line and neutral in an AC system, rather than negative/positive in a DC system.
Polarity, or correct connection of line and neutral, can be determined as a separate live test as shown in this video. Alternatively is can be done as a dead test, usually combined with the continuity test.
Although equipment will still operate normally with line and neutral reversed, UK installations often have single pole switches in the line conductor only. Incorrect polarity would result in equipment still being live even when switched off. A similar problem would occur if the fuse was located in the neutral conductor instead of the line.