You will be lucky if your contractors give you estimates in Excel, so that you could easily compare between the bids. My tiling company insists on Word.doc files. But spend time to understand, where the bids differ. Take some bids from different trades and compare them to the lines that a general contractor gives you. Make sure you feel comfortable with numbers.
And there were a lot of cases when the homeowners had to go back to step 2 at this point.