Apparently we are due a battle sprite update eventually and this is something that really bugs me about battle sprites. Maybe whenever there is an update to them, this can be implemented. When they're level 100 (or level 105), they stop being able to level up any further and it looks like this:
http://i.imgur.com/xJhYK4j.png

However, when most equipment becomes max level, the heat bar turns black and says complete on it like this:
http://i.imgur.com/hYy2gYs.png