Commentary
What Kevin's doing to gauge how much energy the spell used is like seeing how much water is left in a measuring cup after pouring some out.
Someone directly sensing how much energy the spell used would be like seeing some water poured out of a cup, and knowing exactly how much it was without looking at the cup.
Assuming one has the option, Kevin's way is much easier, but it's not without problems.
For example, if Nanase tapped into ambient magic energy while casting the spell, then the before and after difference would not accurately indicate how much magic was used.
Fortunately for Kevin, people don't normally tap into ambient magic unless they have to. It's possible to do so on purpose, but people will naturally use their own magic first if not casting a spell that's in excess of their entire reserve. In that case, they might use very little, or even none of their own magic.
If training without a training wand (a wand that blocks ambient magic uses and provides no power for spells), the trick would be to intentionally use low cost spells. Casting more than one is able to on their own isn't good magic exercise, as the spells will either fail, or be powered by external energy.
Kevin himself functions as a training wand, and he starts by teaching low cost magic. A wizard first builds up their strength, and learns spells they can use on their own to effectively exercise at whatever level they're at.
