Here's how you can tell:
* Articles: The definite article for masculine nouns is "der". So, you would say "der Wald".
* Adjectives: Masculine adjectives will agree with the noun in gender. For example, "der große Wald" (the big forest).
It's important to note that in German, nouns have grammatical genders, which are not always logical. You'll need to memorize them or look them up in a dictionary.