Adjetives declines pretty similarly to typical nouns (with a few notable exceptions in consonant stem declensions).
The noun declension principles are generally this:
a-stems: in vast majority of cases, these are feminines; there are however few exceptions, where an a-stem noun is a masculine (poeta, agricola, etc.) - some of these exceptions come from Greek, some are native.
o-stems - -us ending: in overwhelmingly vast majority of cases, these are masculines, with few exceptions that are feminines (hodus, methodus) - I am not aware of any that do not come from Greek (also do not confuse with u-stems ending with -us, these can be both M/F but decline very differently)
o-stems - -um ending: these are always neuters (unless they are masculines in accusative case that you interpret incorrectly, hahaha).
Adjectives follow these principles during declension, they just do not have any exceptions. Also take note that an adjective can (and in most cases does) have the 3 different forms for the three genders (-a, -us, -um) because they do not have intrinsic gender.
M: servus bonus, agricola bonus, iudex bonus
F: femina bona, methodus bona, arx bona
Considering that adjectives do not have exceptional cases and they have to agree with the gender of the noun (for feminine noun, regardless of its declension type, the adjective will always assume its feminine form), when you read a text that contains noun + adjective, you can often determine the gender of the noun based on the adjective (again, unless it is in a case whose ending does not distinguish this fully, like dative/ablative plural).
Of course, then you have also consonant stem adjectives that decline like third declension nouns and here, you will typically not be able to distinguish the gender except for specific cases and specific adjectives, but that is more advanced stuff when you learn Latin.