During a release we track the status of testing images as we go. This is coordinated in IRC on irc.debian.org #debian-cd.
Link to the test pages should be added here:
Jessie
2016-04-03 Jessie 8.4 /Jessie_r4
2015-09-05 Jessie 8.2 /Jessie_r2
2015-06-06 Jessie 8.1 /Jessie_r1
2015-04-25 Jessie 8.0 /Jessie
Wheezy
2016-04-03 Wheezy 7.10 /Wheezy_r10
2015-09-05 Wheezy 7.9 /Wheezy_r9
2013-05-03 Wheezy 7.0 /Wheezy
Squeeze
2011-0209 Squeeze 6.0 /Squeeze