If you feel that your tax return is not correct and you have already filed the return then you still may have someone else go over the return. If they find that there is a problem with the return then you may file an amended return. You have three years to file a 1040X to make corrections and receive any additional refund you are due.
If you feel that the amendment will result in your having to pay money to the IRS then you should not wait and as soon as your original return is processed (usually when you receive your refund) an amendment should be filed.
Amended returns are not allowed to be e-filed so you will be required to mail the 1040X.
Many tax preparation companies offer what they call"Second Look". Most do not charge unless they find a reason to complete a return for you. If you used a company or paid preparer I encourage you to contact them and advise of your concerns. Most professionals would be happy to sit with you and go back over the information.
I hope this information is helpful to you.