Trusting someone else with money is equivalent to trusting that individual with your life. Based on this, would you really trust your CPA or an investment adviser or a money manager with your life? If not, why would you trust them with your money?
Validate your 1040 (usually if you have wages), Schedule E (rental) and Schedule C (business).
The IRS website does a pretty decent job of listing how to calculate each line on every form in case you have questions.