The IRS relies on a computerized matching system for finding errors and fraud on tax returns. If there is an obvious mismatch between what has been reported to them, and what you report, it is likely they will catch it and make attempts to correct it. Often it will begin with passive aggressive letters sent to the last address they have on file for you. Things that result in you getting refunds and credits are more likely to be acted upon, than things that have little effect. Other things it is fairly unlikely they will check unless it stands out as exceptional in relation to the rest of your tax return, or patterns in tax returns, in general. If you lie and say you bought a new business phone for $1000, it is highly likely they will ignore it. Whereas if you try to claim your cat as a dependent child by making up a tax ID number for it, it is likely you will get caught.