You can check this: http://www.dummies.com/languages/german/identifying-a-german-words-gender/

It is best to learn the gender every time you learn a new word.

There are a few rules for determining the gender of masculine, feminine and neuter.
But, very often, you have to memorize each word's gender.
These explanations helped me very much:

• Gender of Masculine Nouns
  https://www.germanveryeasy.com/masculine-noun-gender

• Feminine Noun Gender
  https://www.germanveryeasy.com/m/feminine-noun-gender

• Gender of Neuter Nouns in German
  https://www.germanveryeasy.com/m/neuter-noun-gender

Game: Guest the Gender
https://www.germanveryeasy.com/m/exercise-gender

Best way is to just memorize the gender of a noun.Here are some of the rules.

Masculine nouns - male person or profession, seasons,days of the week,months,alcohol(not Das Bier),geographical directions,names of mountains,nouns derived from verbs without suffix: -gang, -fang, nouns with the suffixes -er,.ismus

Feminine nouns - female person or profession, most fruits,flowers,trees,most rivers,cardinal numbers,most foreign nouns,nouns with the suffixes -ei,-heit,-keit,-ut,-at,-ung,-schaft

Neutral nouns- diminutives(-chen,-lein),nouns derived from infinitives,towns,countries,most metals,colors.

It's not your fault if you do not understand. Duolingo also does not emphasize it. For example, its dictionary does not type. That's why it does not matter much. For example, look at a real dictionary. It has to learn its definite article, the genitive of a number one together with the noun, and its plural no. shape. These constitute the dictionary shape of the noun.
Der, die, das is an inseparable part of the German noun. If you do not know the preposition, then you do not know the word. They could write the word together, but they do not do it because of the conjugation. Is this comforting? :) One of the languages are not logical. If the language would be built on the logic, we would be in great trouble. If you do not believe it, follow it. :)