The scientific method can be very helpful here. Speculate on what it might be. Present a choice of products / services that have and don't have those features.
If you're creative enough you can come up with a large enough list of what it might be that you feel overwhelmed trying to present them all. In that case do a binary search.
A(1,2,3,4) > B(4,5,6,7) C(1,2) < D(3,4) E(3) > F(4) Turns out people really like feature 3 but they don't even realize it.
If you're wondering how you present a product with only one feature realize what this really means is there is only one feature that is different.