How to know a device is FCC certified?
In the general case you can't.  Unintentional radiators aren't required to be certified.  If tests were done, the results will usually be private between the test house and the company that commissioned the tests.

In the special case of intentional radiators, they do need to be certified, and I believe the certification number must be displayed on the device somehow.  For large boxes, this is usually on the plaque near the power cord that lists the input power required, the fuse size, applicable patents, etc.  For small devices, this may be a small sticker on the back someplace, moulded into the plastic housing, or something.