Checkmate is DC's closest SHIELD like entity in DC. Star Labs, is usually just portrayed as a place where alien tech and villains are taken after the Superhero's have saved the day. I've never seen S.T.A.R. actually participate in protecting anyone, like SHIELD do.
I haven't been reading the title is it an organisation as well as a team?
It's an international task force set up by the UN, they have a giant air base called the Erie and the series focuses on the Alpha Team of Blackhawk agents
The DCU's equivalent(s) of SHIELD isn't nearly as centralized as most of the prominent incarnations of the Marvel Comics organization, mostly because it's not hit over the reader's head that SHIELD is Big Brother with a heart and penchant for getting superheroes killed (looking at you, Ult. Fury). Checkmate is...meh. They don't really ever do much besides call up superheroes and say, "Do this for us and yadda yadda." SHIELD is often the same way, but at least the characters within the organization are interesting and have legitimate arcs.
