"I take it, it's raining" should be accepted. It's not meant to be literal, but it does work in English, and if the German and English have a nearly direct equivalent, that should work. Unfortunately, "Assume" didn't readily spring to mind at the time. Another one should be "I gather it's raining"
From what I understand, commas are used to separate clauses in German. Here, the two clauses are "I suppose" and "it is raining."
Sometimes a conjunction is present, and this is more obvious. For example, "Ich nehme an, DASS es regnet." -> "I suppose THAT it is raining." English doesn't require such comma usage. HTH.