Hi Lukas, deliverability can be tricky to measure. One good way to measure it, is by using seed tests (GlockApps, Mail-Tester, or InboxAlly) to show you where your emails land, and monitoring your domain/IP reputation using Google Postmaster Tools or MXToolbox, along with how well your SPF, DKIM, and DMARC are set up.
Also, monitor engagement trends, if opens or replies start to dip, your inbox placement is probably dipping too. I usually measure the health of a domain weekly using those signals so I have a decent idea of deliverability before sending.