Unless you or they used Turbo Tax you will have to take their word for it. If it was efiled, they should have a printout or email saying it was accepted. I got my Accepted in 20 min. If you have a refund you can check the IRS Where's My Refund but that is flakey and isn't always right or updated.
If your return was Accepted or mailed then check IRS Where's my refund
https://www.irs.gov/refunds/
Oh but you said the IRS is still processing it? It is taking the IRS longer this year to process returns. It won't be rejected. Rejected means they didn't even accept it for processing. So they have it. If they have any questions they will mail you a letter. All you can do is wait. You don't have to file anything else before April 18. Wait to amend because the IRS might catch the missing W2 and adjust your return for you.
