For example, how could I know, as I was walking into a store or sitting in a restaurant, if a mobile skimmer was passing by?
There is no such device on the market, to my knowledge, and for good reason: There is no need for one.
Here are a few things you should know. First, credit card companies are moving away from RFID toward cards with chips that can only be read when inserted into a chip reader. That is, they have no ability to send or receive data via radio waves. That being said, cards that did have RFID tags in them were more secure than old magstripe cards, and no one, to my knowledge, has ever had their credit card information stolen in this way.
If you do have one of those old cards and are still concerned about someone stealing your credit card number, you could simply wrap it in tin foil and stick it in your wallet. That would prevent anyone from reading the card's tag without your knowledge.
—Mark Roberti, Founder and Editor, RFID Journal