A quick demonstration I sometimes use to tease out this distinction: “imagine a six-sided die. Are you imagining it? Good. How many dots are on the side closest to you?”

If the phenomenal experience was the same and the difference is in the description, everyone should still be able to give a number as their answer, even if they have to use different processes to derive the number from their different descriptions. But aphantasiacs will visibly struggle to answer this question, or give a non-number non-answer like “no side is closest to me” or “depends on which way the die is facing”.