How to start practicing backsolving?

This may sound silly - but I NEVER look at the answers before I solve the problem. I appreciate this is stupid - because I could at least evaluate them, but I am too concerned with time.

Now, the more I study the more I realise that solving a problem with 25 quadratic equations is great...but impossible in 2min. I realised I need to start practicing backsolving.

Hence I have 2 questions
1. How does one do it? Where do I start? How do I get into it if I never did it before?
2. How do I quickly figure out if a problem is worth backsolving as opposed to solving?

Thanks!