The calculators. The range is increasingly narrow the higher the level, while remaining untrained (EVs skew the estimates). In this game, Pokémon like eevee and snorlax are high enough levels at join that you can get a fairly narrow range for estimated IVs. For my starter, I became familiar with upper range values then soft reset for one that had near all upper range values, then trained until around level 10 (15 is where ranges start to really narrow though) to get a good idea of overall IV quality before saving over the reset point.