Here's how it works.
1. You file your taxes. Part of your taxes is your signature at the bottom saying that you believe you have filed everything accurately according to the rules.
2. Based on a proprietary formula, the IRS selects a small percentage of tax returns for audit. People with higher incomes or out-of-proportion deductions are more likely to be selected for audit.
3. If you are not selected for audit, that's it.
4. If you are selected for audit, you may or may not be asked about the house sale.
5. If you are not asked about the house sale, that's it.
6. If you are asked about the house sale, you'll probably be asked to prove that you qualify. The IRS instructions may give guidance on what kinds of proof are acceptable, but in general contemporaneous written records are usually enough. With larger items (like yours), I would probably be pretty careful in terms of the records I saved.
7. If the IRS doesn't accept your records or evidence, they can disallow the exclusion and you'd be required to pay the taxes due and possibly interest and possibly penalties.
8. If you disagree with the IRS, I believe there are tax courts where you can take your argument. I wouldn't go there without a tax attorney, so that would get expensive.