good question. in most cases, it's the 9 digits in the lower left of a check. however, for direct deposit in some cases banks use a different routing number. you need to contact your bank. if you use the wrong routing number the direct deposit will be returned to the IRS and they will eventually get around to sending you a check at the address on the return.
how long to get a paper check if DD info is wrong! figure 6 to 8 weeks. maybe longer for many reasons.
Did you file yet? If you filed it is too late to add or change it. Do you want to see what was entered on your tax return? Or what number to enter when you file it?
Banks have a special routing number for ach, etc. You might have to call your bank and ask them for it. Or you can find it on the bank's website.
Here is how to find the routing number on your checks
Do you have Bank of America? If you used Bank of America there are five routing numbers that the bank does not use for electronic transactions. And it depends what state you are in.