Spot hazards as part of planning and doing your work. When you spot a hazard, fix it straight away if you can, before starting work. Use your judgment. If there still is any risk, report it. You also have to report hazards that haven't been managed well enough.
If it is a risk, your first option is to eliminate it immediately if you can. You may also need to inform your supervisor, especially if it needs monitoring.
If you can't eliminate it immediately, you can report it orally to your supervisor or fill in a hazard form for your office.