Well, when you enter "WXXX" and you can only find "DWXXX."
OK, under e-filing go to the call sign reservation system. You can enter a call sign under the "query" tab and it will tell you if the call sign is available. At that time, if the call you want doesn't show up, you can use the request tab to request a change to your call. For commercial stations there is a fee.
If it shows up as "unavailable" but isn't shown as assigned to any station, you may need to have the request walked into the Commission.
When we were preparing to launch our classic rock, I started looking for the call WXCR. It was listed as "not available" but no station was shown having that call.
Some search revealed that a CC station had used that call, but was now using a different call sign. For some reason, the database hadn't caught up. Our DC attorney made a couple of calls and we had the call.