How do I know if a word is masculine, feminine or neuter?

Sometimes you can tell by the suffix. Words ending with -chen, -lein, -ment or -teil are usually neuter. Words ending with -schaft, -ung, -keit, -heit , -in (if it's a person) or -e are usually feminine and words ending with -er, -ich, -ismus, -ist, or -ner are usually masculine. Sometimes its obvious from the meaning (e.g. der Junge means the boy and therefore should be masculine). However that is not always true so be careful (e.g. the girl is neuter- das Maedchen).