The trickiest thing with spotting prepositions is that it's easy to get them mixed up with adverbs and conjunctions—because certain words can do all 3 jobs.
The word as is a good example of this. This slippery little word can function as a preposition, adverb, and conjunction.
In this particular snippet, as is functioning as an adverb in an adverb group (as always) which is then describing the process of the bus turning right: 'right as always'.
(As it happens, the word right is also an adverb in this context. And as usual, what we see here is an adverb group being slippery by splitting in two and one half shuffling further down the sentence.)
Incidentally, of the 3 prepositions we highlighted in the snippet above, only the word at is always a preposition. The other 2, off and past, can function as adverbs and adjectives depending on the context.