The simplest implementation would be to simply have the signals that normally are shown in the tooltip when mousing over the circuit ID in the upper right corner appear in a GUI container called Active Signals that looks virtually identical to what's in the Decider or Arithmetic combinators. A more robust one might include a 'Sourcing Signals' section so it was a bit more obvious when a thing is SENDING signals -- chests sending their contents, roboports sending the number of active logi bots, whatever -- but it's not strictly necessary, merely convenience.
This is supoerior to the circuit ID tooltip because:
1) I only learned about this like two weeks ago and who knows how long it's been like that, which happened because
2) There's nothing visually to suggest the tooltip will display this without knowing beforehand; every other
thing has a tooltip explaining the thing to which it's attached, or at least so many that I can't think of any counterexamples, and3) it's so convenient to just SEE the signals and not have to mouse over something; oh and
4) Many circuit-connected GUIs have a TON of otherwise wasted space. Some examples below.