You need to look for the output of “cmk -D hostname”
It should look like this for your Proxmox host.
hostname
Addresses: 192.168.0.1
Tags: [address_family:ip-v4-only], [agent:all-agents], [checkmk-agent:checkmk-agent], ..some more tags
Labels: [cmk/os_family:linux], [cmk/site:home], [hardware_device:yes], [wichtig:true]
Host groups: some hostgroups
Contact groups: some contact groups
Agent mode: Normal Checkmk agent, all configured special agents
Type of agent:
TCP: 192.168.0.1:6556
Program: /omd/sites/<sitename>/share/check_mk/agents/special/agent_proxmox_ve .. options
Process piggyback data from /omd/site/<sitename>/...