When a standalone DHCP server(ie server not added to the domain) starts,it sends a DHCP information message to its subnet to get information about the domain in which it is located and to see if there is any other authorized DHCP server in the network. If it doesn't get any valid response back either from an authorized DHCP server in the network it starts providing DHCp services to client. If it receives a response from an authorized DHCP server with details of the root domain,it will not provide DHCP services. This detection process is run every 10 mins for an unauthorized/standalone DHCP server
Authorized DHCP :
Only an Enterprise admin can authorize a DHCP server in a domain.This server should either be a domain controller or member of domain. During startup the DHCP will query the Active directory for the list of authorized DHCP servers. If it finds itself in the list, it will start DHCP services. This check is done every 60 minutes. If the DHCP server doesn't find itself in the authorized DHCP server list, it will stop providing DHCP services.