During the year I worked for someone and made some cash under the table (slightly above the non-1099 limit), and as such never received any 1099 from the employer (so far as I know, no forms were filed whatsoever). Though I have other sources of income for the year, I would nonetheless like to declare this income on my return to be safe, not to mention avoid paranoia. My employer doesn't care either way whether I file. I know one must declare all income regardless of circumstances (though I'd prefer not), but I don't know how I should file that income along with the "above the table" income I made. And I don't want to get into a mess with the IRS if/when they come to wonder where extra income for which I have no 1099 (or other forms) came from. Thanks for any advice.

You need to file Form 1040 and report the "under-the-table" income on Schedule C.

This will not raise a red flag with the IRS, but will keep you out of any potential problems with them in the future.