Nouns ending in "Ь" (soft sign) can be either feminine or masculine. They should be always checked in the dictionaries.
However, there are some principles that could help in gender determining.

• professions denoting males are masculine.
Example: учитель (male teacher), царь (Tzar), король (king)

• names of months are masculine.
Январь (January), февраль (February), апрель (April), июнь (June), июль (July), сентябрь (September), октябрь (October), ноябрь (November), декабрь (December)

• nouns that design statuses of women are feminine.
Example: мать (mother), дочь (daughter)

• nouns ending "сть", "сь""знь" are feminine.
Example: новость (news), жизнь (life), подпись (signature)

• nouns ending "жь", "шь", "щь""чь" are feminine.
Example: ночь (night), помощь (help)