title: How to check whether a given Python Environment is a PipEnv
body: <p>currently PipEnv can be used in a directory to check whether we have a corresponding pipenv environment or not (e.g. <code>pipenv --py</code>).
  Is there a similar API to determine whether a given interpreter is a pipenv?
  Wanted to check this before posting an issue on the Pipenv repo.</p>