Used literally, the word "rain" is a noun:
What's that on the window? "” It looks like rain
Idiomatically, it could be perceived as a verb:
"” It looks like [it's going to] rain.
But the most likely part of speech is noun, since there can be other similar nouns in that position. Examples are from Google.
"” Well just looked at the weather forecast for this weekend and it looks like thunderstorms Saturday and Sunday...not the best of camping conditions.
"” Looking at the current weather forecasts, it looks like thunderstorms Saturday night not to mention rain this week and high humidity until the weekend.
"” Lunch is provided. It looks like showers so bring a jacket and a hat. We will be in the large West lot, so the course should be 1-1.25 miles long.
"” I just posted it on the chat schedule tonight. It looks like snow tonight ..... I honestly hope not.
"” Watch the weather and if it looks like frost (which we could still get) then I recommend covering the newly planted perennials with row cover at night to keep them from feeling the chill of the frost.
"” In typical Glasgow fashion we have been taunted with Sunshine through April and now we're in May it looks like floods again!
These seem to be shortened versions of
"” It looks like [there will be] thunderstorms/showers/frost/floods
None of these nouns (except "snow") can be replaced by verbs. So I guess we should conclude that there's an ellipted "there will be" in such sentences.