How to debug if a constexpr function doesn't run on compile time?

For example I have a constexpr function, but I use a runtime variable (not marked as constexpr) to take the return value. In this case, I'm not sure whether the function runs on compile time or runtime, So is there any way to debug?

At first I thinked about static_assert, but it looks like static_assert cannot do this. Then I thought convert the code to assembly code, but it is way too difficult to check the assembly code to figure out.