Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services immediately if diazinon poisoning is suspected and the person has trouble breathing, heavy secretions, repeated vomiting, muscle twitching or weakness, confusion, a seizure, or reduced consciousness.
Emergency dispatchers can explain how to help safely.
Do not delay emergency help to finish decontamination.
If diazinon has been swallowed, contact emergency services or a poison centre immediately.
Do not induce vomiting or give food, drink, or home remedies unless a poison specialist instructs you to.
And for anyone, someone who:
An exposure without symptoms still requires prompt poison-centre advice.
The product concentration, amount, exposure route, and time since exposure determine whether home monitoring or medical assessment is appropriate.
After treatment, seek urgent reassessment if symptoms worsen, return after improving, or include new swallowing, walking, or breathing problems.
Routine follow-up is appropriate when tiredness, mild weakness, vision changes, or concentration problems are gradually improving.
The article also says
Move away from spray or fumes without entering a contaminated area yourself.
Avoid direct contact if the person is covered in pesticide.
After skin exposure, remove contaminated clothing and wash the skin with soap and water if safe to do so.
After eye exposure, rinse continuously with clean lukewarm water.
Every sign above is quoted from the sofidex article on Diazinon poisoning. If you are not sure, err upward — it is always reasonable to be checked.