There are no magic words to hear, but one does need a clearance to enter. A clearance direct to a given point, without specifically stating cleared to enter class bravo, will suffice. By example, if you're told to proceed south along the highway, maintain four thousand five hundred, through class bravo, you're cleared in, without ever hearing the formal clearance.
If some doubt exists in your mind, you should, and are required to obtain clarification. Simply talking to the controller is not enough, but a clearance into the airspace doesn't necessarily require specific wording. If in doubt, ask. A vector into class B with a clearance to a particular point is a clearance to operate in class B airspace.
When operating on a VFR flight plan, the plan remains with flight service unless you are overdue. At that time, towers and facilities at the departure and destination are contacted, as well as facilities enroute or near your route where you may have landed. A ramp check is performed, to see if your aircraft is there. Failing that, the search is expanded. However, your departure ATC facility, and the destination, do not have your information on a VFR flight plan unless you have become overdue on an activated flight plan.
Flight following will not clear you in, as "flight following" is merely VFR radar advisories (no such thing as "flight following;" that's a slang term for VFR Radar Advisories). It's a voluntary luxury service that provides you limited traffic information on a workload permitting basis. Don't count on it getting you into or through any airspace, because that's not it's intent, nor is it a clearance of any kind.
