Check to see if a string is serialized?

I'd say, try to unserialize it ;-)

Quoting the manual :

In case the passed string is not unserializeable, FALSE is returned and E_NOTICE is issued.

So, you have to check if the return value is false or not (with === or !==, to be sure not to have any problem with 0 or null or anything that equals to false, I'd say).

Just beware the notice : you might want/need to use the @ operator.

For instance :

$str = 'hjkl';
$data = @unserialize($str);
if ($data !== false) {
    echo "ok";
} else {
    echo "not ok";
}

Will get you :

not ok

EDIT : Oh, and like @Peter said (thanks to him!), you might run into trouble if you are trying to unserialize the representation of a boolean false :-(

So, checking that your serialized string is not equal to "b:0;" might be helpful too ; something like this should do the trick, I suppose :

$data = @unserialize($str);
if ($str === 'b:0;' || $data !== false) {
    echo "ok";
} else {
    echo "not ok";
}

testing that special case before trying to unserialize would be an optimization -- but probably not that usefull, if you don't often have a false serialized value.

Optimizing Pascal MARTIN's response

/**
 * Check if a string is serialized
 * @param string $string
 */
public static function is_serial($string) {
    return (@unserialize($string) !== false);
}

From WordPress core functions:

<?php
function is_serialized( $data, $strict = true ) {
    // If it isn't a string, it isn't serialized.
    if ( ! is_string( $data ) ) {
        return false;
    }
    $data = trim( $data );
    if ( 'N;' === $data ) {
        return true;
    }
    if ( strlen( $data ) < 4 ) {
        return false;
    }
    if ( ':' !== $data[1] ) {
        return false;
    }
    if ( $strict ) {
        $lastc = substr( $data, -1 );
        if ( ';' !== $lastc && '}' !== $lastc ) {
            return false;
        }
    } else {
        $semicolon = strpos( $data, ';' );
        $brace     = strpos( $data, '}' );
        // Either ; or } must exist.
        if ( false === $semicolon && false === $brace ) {
            return false;
        }
        // But neither must be in the first X characters.
        if ( false !== $semicolon && $semicolon < 3 ) {
            return false;
        }
        if ( false !== $brace && $brace < 4 ) {
            return false;
        }
    }
    $token = $data[0];
    switch ( $token ) {
        case 's':
            if ( $strict ) {
                if ( '"' !== substr( $data, -2, 1 ) ) {
                    return false;
                }
            } elseif ( false === strpos( $data, '"' ) ) {
                return false;
            }
            // Or else fall through.
        case 'a':
        case 'O':
            return (bool) preg_match( "/^{$token}:[0-9]+:/s", $data );
        case 'b':
        case 'i':
        case 'd':
            $end = $strict ? '$' : '';
            return (bool) preg_match( "/^{$token}:[0-9.E+-]+;$end/", $data );
    }
    return false;
}