  The central engine of gamma-ray bursts (GRBs) is poorly constrained. There
exist two main candidates: a fast-rotating black hole and a rapidly spinning
magnetar. Furthermore, the X-ray plateaus are widely accepted by the energy
injection into the external shock. In this paper, we systematically analyze the
\emph{Swift}/XRT light curves of 101 GRBs having plateau phases and known
redshifts (before May 2017). Since a maximum energy budget
($\sim2\times10^{52}$ erg) exists for magnetars but not for a black hole, this
provides a good clue to identify the type of GRB central engine. We calculate
the isotropic kinetic energy $E_{\rm K,iso}$ and the isotropic X-ray energy
release $E_{\rm X,iso}$ for individual GRB. We identify three categories based
on how likely a black hole harbor at central engine: 'Gold' (9 out of 101, both
$E_{\rm X,iso}$ and $E_{\rm K,iso}$ exceed the energy budget), 'Silver' (69 out
of 101, $E_{\rm X,iso}$ less than the limit but $E_{\rm K,iso}$ is greater than
the limit), and 'Bronze' (23 out of 101, the energies are not above the limit).
We then derive and test the black hole parameters with the Blandford Znajek
mechanism, and find that the observations of the black hole candidate
('Gold'+'Silver') samples are consistent with the expectations of the black
hole model. Furthermore, we also test the magnetar candidate ('Bronze') sample
with the magnetar model, and find that the magnetar surface magnetic led
($B_{p}$) and initial spin period ($P_{0}$) are consistent with the
expectations of the magnetar model. Our analysis indicates that, if the
magnetar wind is isotropic, a magnetar central engine is possible for 20\% of
the analyzed GRBs. For most GRBs a black hole is most likely operating.
