I'm working on version 2 of Trenzado, and that means resculpting the other side of his face since his hair will not be covering it (I actually did sculpt it originally and then covered it with hair, though I didn't fully detail it out with wrinkles and whisker bumps and everything). Since I'm doing v2 from a resin cast, there's no way to recover any of that detail so it all has to be rebuilt.
Of course faces are pretty much symmetrical, which is always a bit of a challenge. Here's what I do:
A mirror! This one conveniently stands on its own and came from IKEA but they're available everywhere (you probably already have one). A hand mirror works ok, and is what I used until recently, but this makes it a bit easier to sculpt while looking at the other side. Getting the lighting just right, so both sides are lit evenly, can be a bit of a challenge, but it's doable.
And yep, standard messy desk and references in the background.