Hi Barry, the answer very much depends on the use case and what you are double checking:
If you are being asked for a "secondary validation" of the addresses existence, and it's a postally valid address, you might want to consider an address validation engine.
If the secondary validation request is because the match code (in the US) or result code (Rest-of-World) our geocoder gives you does not meet with certain validation requirements, then there are multiple techniques for tuning the geocoder and cleaning the address data, before re-running the address. Including, interactive address lookup. And if that doesn't work, then where you would try would very much depend on which country we are talking about.
If the secondary validation request is based on a good address match but poor positional accuracy, then I would recommend an online aerial imagery supplier, and who you choose depends on what dataset of our you are running. So for instance, if you are running with HERE data, then Bing would not be a good place to go, rather you would want to go to Google or MapQuest.
If you can give me more details, I can be more specific.
Thanks
Mike