If you have seniority and they are a junior, some juniors do respond well to a senior having more knowledge about the codebase. With them, it can be beneficial to use a tone like "We have library X that seems like it could do a lot of the functionality here, unless you already took a look?" I know it's like 90% of the same but I know people who will just be shellshocked and just blindly say "yes" to any question you ask them, and I don't want a blind "yes" I wanna know the truth :) it also lets then explain why they didn't use it if they have a legit reason because hey, maybe I'm the one who needs to be caught up