Hi Christoph
There are two ways to check it:
1. In the reminder plugin, there is a parameter called BCC Email. You can edit the plugin, enter your email into that parameter. Then each time reminder email is sent, you will received a copy of email and monitor it
2. The other option is going to Events Booking -> Configuration, there is a parameter called Log Email Types. You can select reminder email from there and these emails will be logged. You can access to Tools -> Emails Logs to see these logged email and monitor it
Tuan