No we don't know for sure but the Secret Service does use bomb detectors and at many events there are screening devices people have to pass through. And then there's the stuff they don't talk about that they utilize.

It''s somewhat like with threats. They have databases they each travel with, updated daily (a new download daily is required of all agents on detail) that looks at all perceived threatening individuals in an area (a wide area). And they are expected to know these folks' faces as well as they can. The number of threats you ever hear about (like this convenient news drop yesterday from the ATF about these nitiwits) is nothing compared to real threats from thousands of individuals they simply don't talk about or report to the media.

No one is completely safe anywhere but these dudes are the best anyone can ever hope for.