Currently it is quite difficult to see whether heat exchangers are actually operating. AFAIK, bottleneck works by checking assembler input+output, but maybe it would be possible to make a special case for exchangers (and possibly turbines, boilers etc, but exchangers are most important as they have no other indicator of activity like smoke coming out), maybe based on temperature or water/steam input/output?
See also https://forums.factorio.com/viewtopic.php?f=6&t=52982&p=310528#p310528
