How to identify an existing Enclosure is outdoor or indoor?

For outdoor use could be many different specs and enclosure accessories, starting with Nema 3R enclosure up to Nema4X, but the correct enclosure spec depends on temperature, humidity, elevation, corrosion agent, etc. For example you can't use a Nema 3R in a tropical rainforest nearby the sea because of corrosion, but you can use in a high elevation desert with a space heater.

Manufacturers follow commonly the Ingress of Protection against Dust particles and Water. Indoor enclosures are rated commonly IP42 and Outdoor enclosures are IP65 according to IEC standard 60529. Normal indoor free standing enclosures are commonly can be identified by looks are the knockout louvers but there is outdoor enclosures has louvers as well and it can be identified also if there is cover on to it as protected against direct water contact.

Another priority visual inspection for outdoor enclosures are the door panels, it is commonly design to protect against water. Check if water gasket is correct, it is similar looks like the one used for cars. When you open and close the door panels, the water gasket should contact against the door lip.

If it's a stock enclosure from one of the main manufacturers then it should not be too difficult to find the data sheets to confirm if the enclosure suitable for outdoor use. If it's a bespoke made enclosure then you would need to seek expert advice from either the manufacturer or a control panel manufacturer (The company that made the enclosure not wired it). You would need to open the cabinet and take photos of the type of door seal used etc.

Also you have to remember that mounting an enclosure 'outside' can cover a multitude of environmental conditions, these have to be taken into account. For instance we recently built a cabinet for outdoor use but it was to be mounted underneath a larger loading bay canopy. Read more about Nema enclosure or IEC IP enclosure and you will know to identify it.