If you just have some kind of preference that the actor (or god) has a particular mental state, then a loving God could act like a non-loving God, and you wouldn't be able to tell the difference. It would make no practical difference to you beyond your preference for the entity's invisible mental state.

But asking "how could we tell if God is really loving?" may not mean that. You're not being asked to distinguish between "loving God" and "God that acts loving". Rather, you're being asked to distinguish between "loving God" and "God that doesn't act loving, but is called loving". The two would act in different ways, so the question of good acting doesn't come up.

In other words, if someone claims 'this is the type of thing that would be done by a loving God', how do you determine whether that claim is correct? If someone tells you that God gives people cancer out of love, can you respond "a truly loving God wouldn't give people cancer"?