Two possible solutions:
The simplest solution would be to disable the current workflow and create a temporary test workflow that sends an e-mail under the conditions you require. This assumes of course that you are able to turn off the workflow without affecting the system functionality for other users and risk creating a bigger problem.
The more involved solution would be to build some debug conditional statements into a copy of your current workflow. You can then turn the current workflow off and start the replacement workflow with debug logic. In the replacement workflow you could add something like;
If record-ID is record you want to re-test then
Insert Block that sends e-mail to specific person for approval
Run as usual
You can then manually trigger the workflow and see if the user is getting said e-mail.
One other thought is what happens if a users mailbox is full? If you are using Approval workflow, you can specify the re-send frequency by specifying the Duration per Task. If this is exceeded the e-mail will be resent. This will continue to happen until the users mailbox becomes available again.