This snippet lists all table prefixes from other WordPress installations within the same database:

function get_prefix_options(){
	
	global $wpdb;
	
	$table_names = $wpdb->get_results("SHOW TABLES LIKE '%_postmeta'", ARRAY_N);

	$prefixes = array();

	foreach ($table_names as $table) {
		
		$prefix = str_replace('_postmeta', '', $table[0]);

		if ( !in_array($prefix,$prefixes) ) {
			
			$prefixes[] = '_' . $prefix;
		}
	}
	
	return $prefixes;
}