is "看起来" an adverb or a verb? I've been searching all over the internet and I can't find the answer.
'看起来' means "seem" (look like) and it is a verb, same meaning as '似乎' (seem)
1 : to appear to the observation or understanding
2 : to give the impression of being
He seems very weak = 他看起来很虚弱/ 他似乎很虚弱
he seems quite angry = 他看起来很生气/ 他似乎很生气
看起来 is usually reduced to 看来 in everyday speech
Whereas in English 'look' can be used in a passive sense, very similar to 'seem', I don't believe Chinese 看 is used in that way.
He looks sad. = He seems sad. (How he 'seems' is my impression, not his active intent, unless of course, he is an actor.)
'look' in an active sense is used, for example, with 'at':
Look at this mess!
The character 看 shows a hand 手 shielding the eyes 目. You might do this on a bright day when you look at something in the distance. Chinese 看 is active: 看起来 = 看上去 = look at.
Chinese 似乎(as if, look like, seem) = 好像(looks like)
To use Tang Ho's example: 他看起来很虚弱。We could also write 看起来他很虚弱。
I would interpret 看起来 in 他看起来很虚弱。 as an imperative: 他看起来 = (if you) look at him.
It is easy to see how, in translation, the confusion between passive 'look' and active 'look' could result in 'He seems frail.'
You will often find 看起来似乎 together: 他看起来似乎很虚弱。which you can also translate as 'If you look at him he looks like he is very frail. ('He seems seems very frail' probably would not win any translation awards!!)