I have few concepts of logos for a website. How do I know a specific concept that is finalized, is already used by some other website or not?
In order to avoid trademark violations, there is no way around involving a lawyer. They can assist you in doing a search. Even then, there could be challenges. In the US, at least, a logo does not have to be registered to enjoy trademark protection. Prior evidence of trade using that mark (or one sufficiently similar under a "reasonable person" test) in a similar area of trade constitutes precedence. Defending and losing a trademark battle can be expensive due to costs of implementing changes and the possibility of penalties, not to mention legal fees. I am not a lawyer, so this is just a rough approximation.