What is my obligation to warn fellow kosher observant people about a Restraurant that is claiming to be "Glatt" Kosher, when in fact it has no Rabbinic Supervision at all and the owner's religious observance would not lead you to believe "Glatt" is something he cares about in his home? Does my obligation change any if at one time under a different name he had supervision as a dairy restraurant but had recently reopened in the same location as a meat restraurant?
First thing I'd do is consult a local rabbi, who may have a better assessment of the situation. Also, if the word does need to be spread that the place is not recommended, it will probably be more effective coming from him. (He's also likely to be better protected if the restaurant sues for defamation or something.)
But for theory's sake (or pretending you're the rabbi): To tell people "restaurant ABC has no rabbinic supervision", or perhaps "restaurant ABC has no rabbinic supervision, and local Rabbi X has said on the record that it's not recommended" would seem to meet all of the criteria required for constructive lashon hara:
1.) You're sure the statement is true (assuming you did your homework!)
You or your rabbi might try contacting the webmaster @kashrut.com, who regularly posts alerts of uncertified products and institutions. She has a good sense of how and when this is usually done.