  Prior to the launch of the Swift mission several X-ray line detections were
reported in Gamma Ray Burst afterglow spectra. To date, these pre-Swift era
results have not been conclusively confirmed. The most contentious issue in
this area is the choice of statistical method used to evaluate the significance
of these features. In this paper we compare three different methods already
extant in the literature for assessing the significance of possible line
features and discuss their relative advantages and disadvantages. The methods
are demonstrated by application to observations of 40 bursts from the archive
of Swift XRT at early times (< few ks post trigger in the rest frame of the
burst). Based on this thorough analysis we found no strong evidence for
emission lines. For each of the three methods we have determined detection
limits for emission line strengths in bursts with spectral parameters typical
of the Swift-era sample. We also discuss the effects of the current calibration
status on emission line detection.
