As I understand your question: You are going to assess for everything that can go wrong with fluid and electrolyte imbalance.
Neuro, cardio, pulmonary, GI and GU. Primary assessment in the ed with be cardio, and pulmonary.
The actions I would take:
Put the pt on the monitor (ECG, pulse ox and BP) and get a 12 lead
Start a large bore IV, if I see something Funky on the ECG I may start 2, draw labs from the IV start, may start 1L of NS depending on the ER doc I would be working with. Caution with too much fluids with a CHF or dialysis pt, may want to infusion at a slower rate to start with.
Accucheck (if the BS is really high, this will be useless, most POC meters only go up to 600). Priority is getting the blood to the lab to get I good a BS and K+ levels.
While Im doing all this Im asking the pt questions, assessing their neuro functions, how are they breathing, GI and GU functions.
Get a urine sample... (depending on the pt, stick in a foley, going to need a accurate I&Os, if they are walkytalky I may just get a BSC).
Depending on how the pt is doing resp wise, I will get an ABG.
Im not going to start any insulin, large amounts of fluids, without MD orders. Once the BS starts to get under control 200 - 300 if they are still in the ED, I wll ask the MD for a D5 infusion with the insulin drip, to clear the ketones on a DKA pt.