Department of Homeland Security...Rule of thumb: government agencies with police powers that have "security" in the name are agencies set up by autoritarian/dictatorial regimes to protect the regime from opponents (KGB, Stasi...)
Or the Belgian Veiligheid van de Staat, better known as Staatsveiligheid, meaning Security of the State, which would be Staatssicherheit in German
An a serious note, a better rule of thumb would be to say that in any police agency with "security", the security is not that of the populace but of "The State" as a whole, be it democratic or otherwise.