How will I know that alligation is what they are asking for or it is another equation?
When you are asked to prepare some kind of mixture and have a higher concentration to work from, and a lower concentration to dilute it with then an Alligation can likely be used.
One analogy to help remember it by is mixing cocktails.
Suppose that you have some friends over and one asks you to make her an alcoholic beverage. Let's say a gin and tonic.
Gin is usually 80 proof, which means it's 40% ethanol. Tonic water is 0% ethanol. The reason we mix drinks is to dilute the alcohol and make a drink that is less than 40% ethanol. That's basically what you're doing with an alligation also.
Hopefully, this analogy will be easy to remember. The only other thing to remember is it only works when you are dealing with 2 different strengths. (one higher and one lower) The other thing that trips some people up is they forger that Water can usually be used and is 0%. So, they see a question that says: How much solution "o" 20% is needed to prepare 500mL of 10% Solution "o".