How to handle wrong application of Spells?

So I am a DM for 2 sessions now (Including a training-montage’esque session 0, so 3 in total) and my group is completely new to DnD. Our Paladin has the Detect Good and Evil spell, and apparently uses it rather as a "Detect if there is something dangerous ahead"-Spell, for example to identify a bunch of herbs which were advertised as antidote.

Now for my question: Should I rather push them into reading the spell description again and advise against using the spell in such a manner, or would it be better practice if I just let them waste the spell like that, until they eventually learn it by themselves?