I often test sounds directly in Unity, but by loading a scene and playing the game until I come across the object who’s sound I want to test. Is there a better way to do this, such as testing the object in isolation via the Unity preview or perhaps somewhere in FMOD?
It depends on what you mean by “a better way.”
The advantage of testing sounds in context is that you can hear how they interact with other sounds in your game, allowing you to better judge how well it works within the overall mix and whether and how that mix needs to be adjusted; the advantage of testing sounds in isolation is that its qualities are not obscured or masked by other sounds paying at the same time, making it easier to identify issues specific to that sound that would be harder to notice and isolate when playing it as part of a mix.
Similarly, testing sounds by playing the game allows you to come across sounds semi-organically, ensuring your experience of them is similar to that of a player, helping prevent your preconceptions of how the sound will be experienced from influencing your assessment of it; whereas methodically going through your list of all sounds in the game makes it a lot harder to overlook obscure or easily-missed sounds, ensuring more comprehensive testing.
I’m a quality assurance manager, so perhaps this is just my professional bias talking, but every approach to testing has advantages, but also disadvantages; they’re rarely objectively better or worse than each other. Rather, every method of testing is better suited to some projects than others, and applying multiple different forms of testing to a project is the best way to be thorough.