With the IRS, it is the other way around. The burden of proof is usually on the taxpayer. If a Revenue Agent auditing your tax return thinks you under reported your income or over reported your expenses, it is up to you to prove you did not. It can be difficult to prove a negative, such as trying to prove you did not have as much income as they say you did.
To win an audit, the taxpayer has to come up with some proof to support his tax return. Often, he does not have all of the records, so I have to be creative in producing evidence that will shift the burden to the IRS. Some of the things I use are bank statements, logs, calendars, affidavits, and oral testimony.
When you start out guilty, it can be a long road to resolving the matter, but there are always alternatives and ways to prevail.