I might go with 'arched an eyebrow', or 'arched a brow'. I think most readers would infer the 'skeptical' part.
"He furrowed a brow." Often with this, I connect a line saying the person doing so is staring at who they are skeptical at, and no longer just simply looking at them. I also like what Earp said.
I think furrowing the brow refers to confusion, curiosity, and concentration, not skepticism. Unfortunately I have no better suggestions.
Sceptical is one of the most backwards British spellings I know of. Science, scissors, sceptical... I feel like it should be pronounced septical, but then we'd be talking about waste.
Raises a brow is often a one I use. Using raising as the key word to basically symbolize skepticism. Since you raise suspicion.
I think it's fine just to write 'he raised an eyebrow' and variations there on. I guess the key thing is that when people emote like that they don't move the rest of their face. They are looking unconvinced (or whatever) and then just cock an eyebrow as if to say 'Come now'. So just having that simple description is fine, no need to go overboard on it. Cocked an eyebrow, raised an eyebrow, arched an eyebrow. All the same. Season to taste. I think in these cases where you can't do much with the description the best you can do is just make it short and clean. Wanker, nobber, aubergine; what the fuck is up wit' you people. Now there's an obscure reference for you.
Indeed. Just 'he looked skeptical' is fine. As I said; if you can't make it pretty just make it say what you want it to.