It's not as hard as it might seem. If you are actually held to the contract and you have underbid, you either have a plan to continue the fraud (using substandard materials or illegal labor to save costs) or you will be forced to take a loss or go out of business. If your investors, employees (or their union), or client(s) are unhappy with that, and there is any evidence of intentionally underbidding to be found (and there generally will be, these are decisions made by teams. There are emails. There are notes) those people will have incentive to find it.