  There is strong evidence for the existence of black holes(BHs) in some X-ray
binaries and most galatic nuclei, based on different measuremental approaches,
but black holes aren't finally identified for the lack of very firm
observational evidence up to now. Because the direct evidence for BHs should
come from determination of strong gravitational redshift, we hope an object can
fall into the region near the BH horizon where radiation can be detected.
Therefore the object must be compact stars as neutron stars(NSs), then the
intense astrophysical processes will release very high energy radiation which
is transient, fast-variant. These phenomena may point to gamma-ray bursts(GRBs)
observed. And recent observations of iron lines suggest that afterglows of GRBs
show the similar property in active galatic nuclei(AGNs), implying GRBs may
originate from intense events related to black holes. In this letter, a model
for GRBs and afterglows is proposed to obtain the range of gravitational
redshifts($z_g$) of GRBs with known cosmological redshifts. Hence, we provide a
new method that with a search for high-energy emission lines(X- or
$\gamma$-rays) in GRBs, one can determine the gravitational redshift. We expect
$z_g>0.5$ or even larger, so that we can rule out the possibility of other
compact objects such as NSs, suggesting that the central progenitors of GRBs
should be black holes.
