I think the first sign will be arrests of the political opposition.
Nope the communication blackouts will make the seizures possible.You will not know about it until days or weeks afterwards. How will you protect your neighbors (miles away?) if you do not know of the threat? How can they take our guns if gun owners know about it? They cannot. Be aware of this specific contingency: a communication blackout, including the jamming of ham radio. If the load hits the squirrel cage count on this happening.
The political opposition may come from his own appointees, such as Leon Panetta.
Watch this one very, very closely.