What is a fail-safe method you can use to find reasoning for a step in a two-column proof if you're not sure of what answer to give?
There is no such thing. There is no fail safe method that will work for all possible two-column proofs.
The methods used to prove things in geometry are very different from those used in algebra, which are different from those used in arithmetic and in calculus and in trigonometry. No one-size-fits=all, never fail method could possibly cover all possibilities.