Roslyn - Is symbol implementation of interface?

Sure its possible.

Just for your Dispose example:

var disposeMethodSymbol = ...
var type = disposeMethodSymbol.ContainingType;
var isInterfaceImplementaton = type.FindImplementationForInterfaceMember(
            type.Interfaces.Single().
            GetMembers().OfType<IMethodSymbol>().Single()) == disposeMethodSymbol ;

But if it for general use you need to write it more generally, use AllInterfaces and not Interfaces and sure not use Single.

Example:

public static bool IsInterfaceImplementation(this IMethodSymbol method)
{
    return method.ContainingType.AllInterfaces.SelectMany(@interface => @interface.GetMembers().OfType<IMethodSymbol>()).Any(interfaceMethod => method.ContainingType.FindImplementationForInterfaceMember(interfaceMethod).Equals(method));
}