Check if a string is UTF-8

Is it possible? I want to check if a string is utf-8 encoded.

UPDATE: So, the problem is: I h开发者_开发知识库ave a php webservice. This webservice has a method that receives a string. I'm trying to pass a VB.NET JSON string to this webservice. json_decode always returns an error, saying that the string is not utf-8. If I pass the string and convert it using PHP utf8_encode, the accents get wrong! How could I solve that?