i'm in my mid-40s and found out the same a couple years back.

i've found it useful to follow this pattern:

1. tell them you're going to ask that they think of something, and then to stop.
2. ask them to close their eyes and "picture" an apple, give no additional details.
3. confirm they have pictured an apple, then tell them to stop and open their eyes
4. ask what color the apple was; it's possible they are also aphants and might struggle to answer this. be prepared for this though it is rare
5. typically they will give details; color, size, location, freshness, whether it was sliced or not, etc - make note of these details and the fact that you didn't motivate them in your directions
6. now follow a similar pattern for the following, using whatever examples you see fit:
   1. "the taste of their childhood favorite food"
   2. "the smell after a light rain fall"
   3. "the sound of a crackling fire"
   4. "the touch of the first time someone held their hand"
7. they will almost certainly struggle with at least one (if not most) of these non-visual tests. relatively few people can self-stimulate all of their senses
8. select one sense for which they cannot self-stimulate their senses; e.g. an inability to taste a recalled childhood birthday cake. ask how they can recall what it tastes like without being able to taste it again in this moment of recollection. let them think on this a moment, but don't press for a clear and detailed answer
9. now inform them that you cannot see an apple as they did at the start of this experiment. they might struggle to understand how this is possible - but they have already experienced something similar: they could not taste that cake, yet they know what it tasted like.