Science

I would, like many wise people say, practice reactions, but being a student myself, I know most of us don't spend time looking at reactions, let alone practising them:) In that case, I recommend understanding at least general organic Chemistry including topics like isomerism, stereoisomerism, electron affinity, arrow pushing, hybridisation and, in some problems, MOT. Looking at the reactant, you can eliminate which reactions it couldn't undergo. Looking at other reactants, you may eliminate them further. Then looking at the catalyst or conditions of reactions, you can identify which reactions are possible. Till here, you would need to practice at least a few reactions or have a good memory. It's okay if you don't have it. You can also eliminate them by using your general organic Chemistry knowledge. Draw the structures of reactants and catalysts (if possible, draw structures in such a way that can be visualised in 3D), and for most cases, find out the electrophile or nucleophile and attacking centre, then draw the arrows indicating electron movement. Comparing the stability of products, in many cases, would lead you to your answer.