There's 2 ways to see which engine is first:
1.  When you go to a serial preset the first engine will always be what shows bright when it's recalled. If you're editing that preset and get confused you can always save and recall it.
2.  Or, if you switch engine order(press shift+engines) it will always show the first in line as bright after a switch.  So you could switch twice if you forget which one is first.