Is 晚 a verb or an adjective in the sentence 我 晚了一 个半小?
晚 in "我晚了一个半小时" is an adjective. 

"我(是)晚了一个半小时" = "I (am) late for one and half hour"

The verb 'be' (是) is omitted in Chinese

The verb 'be' cannot be omitted in English. "I was late for one and half hour" cannot be reduced to "I late for one and half hour". 

In "我去晚了一个半小时", '去' is a verb and '晚' is an adverb