I think "look it up" is more natural in this case, but both work. If you did not know for sure is something is an actual word, the first sentence is better. Is caloriduct a real word? I don't know. Let me look for it in the dictionary. What is the meaning of the word rapport? I'll look it up in the dictionary.

They are both correct. The first one is more generic e.g. 'I have lost my book. I will look for it in my house' The second one however, is a phrasal verb, which means 'to locate in a list or index'