I use character guides to find out what the best items for a character is, but often there are three categories. Items if you are against AD heavy champions, items if you are against AP heavy champions, and balanced. How can I tell if the champions im up against are AP or AD? Do I look at their items?

Alt+Tab and google them

One way to tell is by looking at their items. Another way to tell is the death recap. If most of their damage was magic damage, than they are an AP Champ. If it was physical, than they are an AD Champ. Some champions can go either way, so looking at their item builds helps also. If Master Yi is building a Rabadons and other AP boosting items than grab some MR. If he has a Blood Thirster, grab some Armor.

could i try checking by clicking on them during the game to see all their stats in the top left corner? ive tried that once but it seems that everyone had more AD then AP, even magicians.

AD is the amount of physical damage that your autoattacks do. All champions have AD naturally and gain additional AD as they level up. The AD you're seeing is their total AD: base plus bonuses.

AP is different. All champions have 0 base AP and only gain AP from items, masteries, and runes. (A handful of champions like Galio and Karma gain AP through their abilities.) If you check a champion's stats and they have any AP at all, that must mean that they're doing something to increase their AP and it is probably safe to assume that they are at least partially AP.

When guides say "build x against AP and y against AD", they don't mean AP or AD. They mean magic damage and physical damage; it's just easier to write AD and AP, and for most older champions building AD leads to them dealing mostly physical damage while building AP leads to them dealing mostly magic damage (Sion and Master Yi are two prime examples).

These days, though, looking at a champions items or stats doesn't always help you when deciding whether to build more armor or magic resist. For example, Darius' passive, Hemorrage, deals magic damage but scales with bonus AD. Poppy's Devastating Blow scales is an "on next attack" modifier that scales with total AD but converts all damage dealt (including damage dealt by your basic attack) to magic damage. Building only armor against Darius or Poppy isn't a very good idea, even if they both build a lot of AD items. Jax (Leap Strike) and Akali (Crescent Slash) are two examples of champions with abilities that scale off of AP but deal physical damage.

The only way to know for sure what damage type an enemy primarily deals is to look them up ahead of time, then compare the scaling on their abilities to what they are building. That said, almost no abilities scale with AP but deal physical damage (Akali's Crescent strike is a notable example of an AP scaling ability that deals physical damage), so if the enemy builds AP chances are they're going to be magic damage based.

Something like Akali's crescent strike is a good example of an ability that scales from both attack damage and ability power, but still does just one type of damage regardless of what items it's being boosted by. It'll always do physical damage, even if the player builds Akali only with ability power items. Another example is Ezreal's ultimate, Trueshot Barrage - it scales both off attack damage and ability power, and deals magic damage.

I feel that if you're in doubt of what type of damage they do, build some armor or magic resist to mirror the type of items they're building, but emphasize health. Health is relatively cheaper compared to armor/magic resist now, and it's become a pretty efficient buy for survival.

LOOK at their items. If they have items granting AD they AD. If they got tanky items, they tanky. if they got AP items, they are AP. if they got support items they are support.

but in honest, just learn more about the game too, like look up champ page on LOL wiki and see if their ratios are AD or AP and stuff.

by not being some dumb nigr azz jewnube