Through my studies I have noted a general rule that can help people with knowledge of Spanish differentiate between nouns of the 2nd and 3rd declension. Differentiating between these two declensions is particularly important seeing as the 2nd D. singular genitive is so similar to the 3rd D. dative/ablative plural, which can easily lead to mis-translations.
The trick is that Spanish words that are descended from 2cnd declension nouns mostly end in "o", here are some examples:
On the other hand, Spanish nouns that are descended from 3rd declension nouns mostly end in "es" in their plural forms, here are some examples:
So far, I have only come across 2 exceptions to this rule - 2 third declension nouns whose descendents end in "o".
Maybe this is common knowledge and is already explained in Spanish textbooks of Latin? But anyway, I thought it was interesting and useful.