There is no "polarity" on an AC circuit. Just Hot (power) and Neutral (reference). The "polarity" reverses at the frequency of the generator (usually 60 Hz (times/second) in the US or 50 Hz in Europe).
To answer your question, if the wiring of your outlets are reversed, they were that way to start with. What fried your cable boxes was a voltage surge either on the power supply or the cable itself. This is why it's a good idea to have your electronics connected to a surge suppressor although some cable companies don't like that if the cable signal is weak because they don't know what the suppressor is doing to the signal.
It's easy to use one of the circuit testers recommended by the above responses to check the wiring of each outlet in your house. The testers are cheap and some power companies offer to lend them for free. Once you figure out which outlets are suspect it's generally easy to refer to any handyman website and learn the white wire goes to the wide prong (usually the silver screw) and the black wire goes to the narrow prong (usually the brass colored screw). Green wire is ground and goes to the base (usually tinted green) prong at the bottom of the outlet. Make you turn off the circuit breaker or pull the fuse before you work on the outlet. You can use the circuit tester to verify the power is off. Otherwise call an electrician to fix the suspect outlets.
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