BACKSLASHEDP character
BACKSLASHED? character

Outputs TRUE if character was originally entered with a backslash (\) before it or between vertical bars (|) to prevent its usual semantic meaning. Outputs FALSE, otherwise.

That is, it outputs TRUE if only if character is a backslashed space, tab, newline, or one of ()[]+-*/=<>":;\~?.

SHOW BACKSLASHEDP "a
false

SHOW BACKSLASHEDP "|(|
true