Hii Mukesh!
The trick is very simple. Once you get a question, read it carefully and look at the details provided. Then, look for what is to be found. Then, you need to analyse the situation given in the question and not down the important values. Now, think of the possibile formula which can be used using the values provided to reach to the value to be found. And you get the possible formula for the question. But, this requires practice. The more you practice, the more you will be good at it.
Hope this helps.
Good luck!