For dynatrace managed go to CMC and click on cluster (if you have multiple one you have to click on each one). There you will see cluster version.
ActiveGate versions you can find on deployment status, go to tab ActiveGates. Each of them will have marked version and info if upgrade is available.
To see OneAgent version you have to go to each host. There on properties on top you will see agent version on host. After upgrading host you have to restart all processes to allow them upgrade as well. This is why it is possible to have different version of agent on process and host. Such process will be marked as restart candidate.
In addition to Sebastian's correct answer:
The deployment status view also gives you a list of your hosts, including the OneAgent version. You even can filter for outdated OneAgents, etc.
Is there any way to get the Dynatrace Managed Server version from the command line?
We tried: /opt/dynatrace/server/dynatraceserver --version
Output: Dynatrace Watchdog Version: 126.96.36.19991115-105048 - built 2019.11.15 10:50:48
But our installed version is actually: "188.8.131.5291115-130238".