At an outstation, the key for me is to track the inbound equipment. If I can do that with reasonable surety, I will not go to the airport until the delayed departure time. In your example, I'd look at the entire DL SNA schedule and in doing that find that Compass operated E75s only come in from SEA (SLC/ATL/MSP/etc are all on other equipment). The morning SNA-SEA has already departed thereby eliminating that E75 as a possible replacement for your delayed flight. This means that the very first E75 available for your flight will be the morning SEA-SNA (inbound DL5737). Having that knowledge, I would simply track the inbound flight and make sure I'm at the gate a few minutes prior to its arrival. Note I did not say at the announced delayed departure time, but rather a few minutes prior to the arrival of the inbound. Most airlines** try and make up as much time as possible so if the inbound arrives earlier than expected, they will depart earlier than announced. Obviously the airline could do something unexpected like bring in an E75 from somewhere else, but the chance of that is remote enough (IMHO) as to discount it as a possibility. If I had done this same analysis and found an E75 arriving from SLC around the same time as the original departure time, I would be at the airport at scheduled departure time. They could easily decide last minute to use that plane to go to SEA on-time and delay a different flight. Obviously, all of this goes out the window at a hub as equipment swaps are feasible and frequent.
All the standard warnings apply - YMMV, DYODD, etc. This advice is worth the price you paid for it.
**I say "most" airlines because as an AA regular now, I've found that AA sometimes will stick to an announced delayed departure time regardless of when the inbound shows up. Recently at DFW, an inbound a/c was coming in from Florida and there was a large weather system en route. The flight aware track had them going a LONG way around the storm, hence our delay. The pilots were able to find a path through the storm and they ended up arriving at the gate around an hour before our delayed departure time. Very annoyingly, gate agents sat there for 30 minutes and started boarding 30 prior to the delayed time - UGH.