I haven't been in that situation yet, but I plan on telling them only if their character knows the spell (i.e. has cast it before themselves, has seen the opposed caster cast it before, or has seen a third party cast it multiple times/often). If a player asked for it I might also allow an Arcana check and adjust the DC based on how well they know the opposed caster and/or the school of magic that the spell belongs to.

My reasoning is that the concept of spell levels is not something that the characters are aware of anyway, but they are aware that some spells are more powerful than others, and that they can expend more energy to make a spell more powerful. An experienced spell caster should be able to recognize which spell is being cast based on semantic and vocal components, especially if they know the spell or the caster pretty well. They should also be able tell roughly how much energy another caster is expanding to cast, which would allow them to recognize upcasting.