1) Sarah looks ill . < Right now< Sarah doesn't appear to be okay (or well). 2) Sarah is looking ill . < This could mean right now but it also could mean she has not appeared well "recently" "lately" or "these days" or "as of late" < which means "recently". I hope Sarah feels better soon! ha ha

I agree with ryabina. Apart from the techincal terminology, they share a similar meaning and are interchangable in the example you quote.

We can use verbs that describe the way we feel physically in Present Simple or Present Continuous. I feel / look ill (now). I am feeling / looking ill (now). My legs hurt (now). My legs are hurting (now). Present Continuous is more frequent in spoken English.

You can use the present simple or continuous to say how somebody looks or feels now: 'You look ill today' or 'You are looking ill today.' 'How do you feel now?' or 'How are you feeling now?'

ill is an adjective so sarah looks ill means she is sick and it's obvious from her face maybe it's pale sarah is looking ill: it's not acceptable to say it asif you want to say she is looking ill now and after afew minutes she will be fine also after the verb we use adverb to explain how the verb is done she is looking steadily at the ceiling: means she is looking now.