The dangers may be from live electrical equipment, other people, unstable materials/structures, and so on
Check for a response using your voice, tap the person's shoulders, pinch them. With babies, flick their feet. At this stage, you may call for help.
To check the airways, tilt the head back and look for obstructions. Do not attempt to reach in an pull out an obstruction, as this could push it further in.
Hold your cheek close to their mouth, to verify if they are breathing. Even a faint breath is better than nothing.
Check for severe bleeds by unzipping clothing, feeling clothes for dampness (sticky feel of blood), see whether they have pale gums. In a rare case (such as a severed femoral artery, in the groin) the bleed may be too rapid for you to control it.
Only call the emergency services (999 in the UK, 112 across all of Europe) on completion of the primary survey. They will need to know the accident location, gender of the patient and how many people are involved.