If you would look it up, you'll find it is a noun.
The sentence does not sound natural to me. The use of "would" here is unnecessary, but it seems particularly common for German speakers learning English.
Your first alternative sentence uses "would" in the same way, and so it also sounds unnatural to me:
If you would look it up, you'd find that it's a noun.
Your second alternative, however, sounds just right:
If you looked it up, you'd find that it's a noun.
The "would" (abbreviated into you'd) is now in the dependent clause of the conditional. The word "that" (like the word "then") is optional and does not affect the meaning.
All the sentences below are grammatically-consistent ways of expressing the same idea:
If you looked it up, you would find it is a noun.
If you were to look it up, you would find it is a noun.
If you look it up, you will find it is a noun.
Note that the last one contains contains no "would" at all.
Please use more precise headlines for your questions. Glancing at the title, I thought you were seeking some general information about conditionals. – CowperKettle – 2016-05-01T06:11:26.623
I guess it's something like "if you should look it up, you'll find it as noun" where would should and other modals work fine here. – Devin Hudson – 2016-05-01T06:57:47.387