As a D&D5e newbie player, I'm getting to grips with all my skills and abilities and when to use them.

One thing that's come up is when we encounter monsters ... what do you roll against when trying to gauge a monsters weaknesses, so we can use fire, water, light, etc, against it?

Do different types of monsters require different rolls to understand what their possible weaknesses could be, eg Insight, Nature, Arcana, etc?

Some players round the table are GM's themselves and very knowledgeable about what to use against monsters, but their characters wouldn't have a clue, having never encountered them before, so it's difficult to find the right balance as a newbie who doesn't want to buy a monster manual or use an online search!