Identifying a fire door in an existing building is a common problem and a surprisingly difficult one, because the features that make a door a fire door are mostly invisible once it is hung. There are reliable indicators and unreliable ones, and the honest outcome for many older doors is that the rating cannot be established without investigation.
The reliable indicators
A certification label or plug on the top or hinge edge, naming the manufacturer, the scheme and the rating. Intumescent seals in grooves in the leaf or frame. A self closing device. A frame with a substantial stop. Handover documentation naming the door. Any of these is real evidence, and the label is the best of them because it is traceable back to a certification record.
The unreliable ones
Weight, which varies between constructions and is a poor guide. Thickness, for the same reason. The presence of a keep shut sign, which tells you what somebody believed rather than what the door is. A solid sound when tapped, which distinguishes solid from hollow and does not distinguish a fire door from a solid ordinary door. None of these should be used to conclude that a door is rated.
Where the sticker fits
A fire door sticker is the keep shut or keep locked shut sign required on the door. It is a management sign and not a certification mark: it tells occupants how to use the door. Certification markings are on the edge and are not the sticker on the face. A building with signs on doors that are not fire doors has a signage problem, and a building with fire doors and no signs has a different one.
When the evidence is absent
Record the door as unknown and treat it according to the risk. Investigation can involve opening up to look at the core, which is destructive, or replacement with a certified doorset, which is often cheaper than establishing the truth about an old door. What should not happen is an assumption in either direction, because assuming a door is rated leaves a gap and assuming it is not leads to replacing doors that were fine.
Questions people ask about how to tell if a door is a fire door
Does a fire door sticker mean the door is fire rated?
No. It is a management sign telling people to keep the door shut. The certification evidence is the label or plug on the edge and the documentation behind it.
Can a joiner tell by looking?
An experienced surveyor can identify most certified doors from the markings and the seals. Where there are no markings, nobody can tell by looking, which is the point.
Is an old door without seals automatically not a fire door?
Older fire doors were sometimes built without intumescent seals under the standards of their day. That does not make them compliant now, and it does mean the absence of seals is not by itself proof that the door was never intended as one.