Doing some cursory Googles (I don't use at much myself either, except when upgrading salt minions) it seems like atd doesn't log very much at all, unless you run jobs with the -m option. Another suggestion was to wrap the command being run with at in a wrapper to syslog:
ex:
#!/bin/bash
logger -i -t mycmd Starting
/bin/somecommand
logger -i -t mycmd Completed
exit 0
It looks like you're out of luck for determining what happened in the past, but going into the future either the -m or wrapper options might work for you.