Oh, here's something you might try. If you haven't yet filed a 2019 return, the "Where's My Refund" tool "might" have info on your 2018 return if you are due a 2018 Federal refund.
The IRS refund FAQ page says the tool is supposed to have "information on the most recent tax year refund we have on file for you."
If you haven't filed 2019, then it can't hurt to try, but if it doesn't find your 2018 information, that doesn't necessarily tell you for sure whether they received it or not. The best way to tell for a past-year return is to ask them. I'd certainly want to talk to them to confirm first before resending one, which could cause issues if they already have it.
If you only recently mailed the 2018 return, it likely won't show up. Normally it takes at least 4 weeks for a mailed return to show up in the IRS tool, but any manual procedures are likely further delayed with the Coronavirus situation. Just saying, since you didn't say "when" you mailed it--last year or recently.
So it only helps you IF it shows up in the tool. If it doesn't, you still won't know for sure.
IRS "Where's My Refund"
https://www.irs.gov/refunds
