As an adverb, "quite" can be used meaning both "fairly/pretty" and "completely/absolutely". I have difficulty understanding when it is a synonym of the former adverbs and when a synonym of the latter ones. Are there any clear-cut rules?
This is quite an interesting topic.
I believe the best way to determine the meaning is by context, but this is easier said than done.
In American English quite with gradable adjectives often means something like 'very', not 'fairly/rather'.
Source: Michael Swan's Practical English Usage, 489.1.
When American speakers say quite, they usually mean "very": We've examined the figures quite thoroughly. In British English quite usually means "fairly": The film was quite enjoyable, although some of the acting was weak. Speakers of British English sometimes use quite to mean "very", but only before words with an extreme meaning: The whole experience was quite amazing.
Source: Macmillan Dictionary usage note
When we use quite with a strong adjective it means the same as absolutely:
The food was quite awful. = The food was absolutely awful.
As a child he was quite brilliant. = As a child he was absolutely brilliant.
Source: British Council