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Google WebmasterTools can give you some reports that might be helpful - but it is down to you to interpret what they say.
However, you must put the file in the root of the domain that it needs to apply to, the disallow URLs must contain paths and filenames and must NOT include domain names (unless that's actually the name of a folder).
You might not need to end the disallow statement with the trailing slash, because having the trailing slash in the disallow means that /folder isn't blocked. That may, or may not, be a problem.