In some language like c, java etc keyword is specified so the compiler will detect the keyword in the lexical analysis phase.
While in some language like FORTAN keyword are not specified, it uses lookahead operator ( follow certain pattern before certainty of token for the keyword). These patterns are specified in Lex language (lexical analyzer generator).
Like for keyword the pattern are defined as in LEX language:
IF / \ ( .* \ ) { letter} here for certainty of IF as keyword lookahead (lookahead operator '/ ' ) certain pattern.
for more information follow book compiler design by ULLMAN.