SyntaxLength["string"] finds the number of characters starting at the beginning of a string that correspond to syntactically correct input for a single Mathematica expression.
SyntaxLength effectively returns the position of a syntax error, if one exists.
If SyntaxLength returns a position past the end of the string, it indicates that the string is syntactically correct as far as it goes, but needs to be continued in order to correspond to input for a complete Mathematica expression.
See The Mathematica Book: Section 2.8.13.
See also: SyntaxQ, $SyntaxHandler.