Use Faerie Fire as a surrogate for Fireball
Advantages:
- Faerie Fire is non-lethal.
- Like Fireball, Faerie Fire uses a Dexterity saving throw.
- Targets who fail the save light up, making it easy to determine who failed.
- Faerie Fire is a 1st level spell, so your wizard should have significantly more of them available for training on any given day.
Disadvantages:
- Faerie Fire targets a 20 foot cube (an area effect, but quite a bit smaller than a 20 foot radius fireball).
- Faerie Fire only has a range of 60 feet vs fireball's 150 ft.
- Faerie Fire is a Druid spell, and not on the Wizard's spell list. (Curse you 5e!)
A few disadvantages, but as a training exercise for Dexterity saves, maybe they're acceptable, although having to hire a druid probably makes this a non-starter...
Alternately you could use Darkness or Fog Cloud, but they are harder to judge since they don't require a Dexterity saving throw.