To check if a client has uploaded and whether there's an issue with the file, go to "My Incoming Ads" tab and locate the upload. If the status is "Review Warnings", it means the file contains warnings but can still be delivered to your team. If the status is "Review Errors", it means the file contains errors that will need to be fixed - in order to meet your publisher requirements - before it can be delivered to your team.
To view the errors on the file, click on the job status and it will take you to the job page. From there, click on Review Errors and it will open the pre-flight report listing all issues with the file.