If a system takes action, or affects something, those effects, or lack of, are notable. For an electronic system it's simple - it is powered on.

[edit] You can also consider the transient and steady states:

[1] steady state: https://en.wikipedia.org/wiki/Steady_state

[2] transient https://en.wikipedia.org/wiki/Transient_state

[edit]

My comment about the start doesn't actually address your concern about the end of the transient state. I think there are options. Any observable result can be considered the output of the system.

[edit]

Yoda would say "do or do not, there is no try." I prefer "there is only trying."

[edit]

i need coffee >.<