Now, How do you know if a champion builds tank, critical strike. Tank and Crit are the only problems i keep forgetting. I know that Shaco, Yasuo builds crit, But how do you build champions if they are tank or crit, or AP, or AD. Thanks to all who replied.
its all about how well a champion's kit synergies with the items in the game. If your champion can utilize crit items well then get them. For example, you wouldnt build crit on someone1 like malphite lol.
Is a champion a marksman? 99% of the time they build crit. Does a champion have crit in their kit? 99% of the time they build crit.
AP shaco is better than crit shaco though. And raw AD shaco is better than crit shaco. But crits make everything viable. even AD azir.
there are many ways to tell, sometimes there are multiple builds
What you need to do first though is figure out what a champions job probably is by looking at their kit.
you can look at how pro players build things by going to probuilds, just google a champion name and add "probuilds" but be warned, those builds are for professionals with good teams backing them up.
Every champion has reccomended items and they are usually on point.
Champions that build Tank usually have a % stat associated with the defense. For example, Volibear heals for 30% of his health and has a % Bonus Health Ratio on his W. Obviously, you should be stacking HP on that champion to maximize on these percentages. This applies to all stats really. A champion that has a % ratio in their kit tends to get more bang for their buck relative to every other champion in the game for buying these same stats. To rephrase, since Volibear has built in Health Ratios, he is a champion you should go out of your way to build Health on compared to other champions.
On the other hand, some champions just don't have these % ratios but still need the defense. Think Jarvan or Wukong type champions, those with decent flat defenses but no scalings like Volibear has. In this scenario, you need to build for the situation. Wukong primarily builds AD Caster for example, however once team fights roll around, he explodes far too quickly even with strong uses of his Decoy. Maybe he gets blasted by enemy AP and needs the Banshee's Veil to knock some of that damage off. Maybe the Enemy ADC locks on to him consistently and Randuin's Omen will help him make full use of his ult. Maybe you need flanking power on top of defense and opt for a Dead Man's plate. This applies to every champion in the game that don't already have % scalings: if you're in a situation where you're dying too quickly to be useful, build defense based on the situation.
Crit can follow a similar logic to Tankiness. Shaco and Yasuo have Critical Strikes interlinked with their kit and naturally you want to complement that. The more useful way to tell if a champion wants crit is based on these factors:
ADCs for example are normally explicitly chosen to be the team's damage source, especially against Tanks. They auto attack by far more than almost any other champion not named Master Yi, and due to their usual inability to build for Utility like a Tank or Mage can, often their only option is to optimize their Damage. Compare this to Veigar who loads his firepower into his abilities, or Gnar who is primarily picked for his Ultimate and lane control than to be a Damage Dealer, it's obvious to tell who benefits the most from Critical Strike Chance between those champs to, say a Tristana.
Most ADCs build crit, but it's usually for champions who are AD and auto attack a lot. Not saying all of them, but as you'll see a lot of them. I'll make a quick list, though probably will miss a few.
Main crit users:
Casters that don't often build it, but can:
I didn't include off-meta builds or roles like AD Kennen etc. Also didn't include any champions that only get crit through 
since the crit is getting removed from it on the next patch.
