You need to filter leads who were sent an email by time zone to confirm the Recipient Time Zone feature is working correctly for all leads.
The "Was Sent Email" and "Opened Email" filters do include the constraint "Date of Activity" but it is not that granular. You can get granular to the minutes, but you can only select "in past: 5 minutes" for example, not a specific time or date range. One workaround would be to View Results for the Email Program and filter for the "Send Email" activity. You could export the results to a spreadsheet to make sure people are being sent emails at the correct time.