I received an additional 1099. I already filed my 1040 AND received my refund. I have not yet filed my state return. I need to mail in my 1040x, but will the IRS let me know when they receive it and if it is accepted? I am guessing I should wait until I know this before filing my state tax return?

“I need to mail in my 1040x, but will the IRS let me know when they receive it and if it is accepted?”----> Correct; if you're due a refund, the IRS will send it to you after it accepts your amended return. If you owe tax, you should send the amount owed to the IRS along with Form 1040X. If interest or penalty is due, the IRS will bill you. Aas you can see, you should file Form 1040X, along with a new Form 1040, plus any other necessary schedules and forms or etc. Amended tax return needs to be mailed to the IRS on paper for manual processing. It usually takes return about 8 to 12 weeks, forThe IRS, to processes an amended return. However, this process can take longer during the IRS's busiest times.
“ I am guessing I should wait until I know this before filing my state tax return?”---->Correct; unless you amend your fed return, you can’t just amend your state return. Amending a state return also may alter your tax liability, causing you to owe additional taxes to the government. I guess you may contact your state's Department of Revenue to obtain the correct form to amend your state tax return. The IRS website provides a link to each state's Department of Revenue. This link allows you to obtain the form you need.