How do I check an AI estimate against jobs I already closed, so I know where it can be trusted?

Owners tell each other to "run it against known numbers", which is right, and then nobody explains the method. Here it is, and it takes an afternoon once.

Pick ten jobs you have closed and know the real final cost of. Choose a spread — small and large, simple and awkward. For each one, give the AI only the information you actually had before you started. This is the part people get wrong: if you include anything you learned during the job, you are testing its reading comprehension, not its estimating.

Write down its number next to your real number. Then ignore the average error, because the average is the least useful figure in the table. Look at where it missed and by how much, and look for the shape. Most owners find it is reasonable on straightforward repeat work and wild on anything with access problems, unknown existing conditions, or heavy custom fabrication.

That shape is the actual deliverable. It tells you where an AI first pass saves you real time and where it will quietly cost you a job. Redo it once a year, and any time your supplier pricing moves hard.