1. Enable the GoogleImages tab gadget in your preferences and use it to check for images that have been previously published elsewhere. Use {{No permission since}} or {{Copyvio}} where applicable.
2. Look at the file name and description. Many files marked as unidentified are actually identified and just need appropriate categories. Google Translate may be useful if the description is written in a language that you don't understand.
3. Check for usage in galleries or on other projects. If the file is used, there will be File usage on Commons and File usage on other wikis headings on the file description page.
4. Look for clues among the uploader's contributions and upload logs, particularly around the time the file was uploaded, date and time from the metadata of the previous and next photo etc. A link to the uploader's contributions can be found in the File history section on the file description page. A link to the user's logs is at the top of the contributions page. A link to contributions on other projects can be found at the bottom of the contributions page.
5. If the file was transferred from another project, look at the original uploader's contributions and upload logs.
6. Look for street signs, house numbers, shop shields, company names or similar clues in the image itself.
7. Any of the popular webmaps (OSM, Google, Bing) in conjunction with street level imagery providers (Mapillary, OpenStreetCam, Street View) can often be used to pinpoint a more exact location once a general location has been established.

If you are able to identify the subject, edit the description to better explain what the subject is. Remove the file from this category and place it in the most specific applicable categories. If you have figured out exactly where a photo was taken, add {{Location}} to the file description.

If you are not able to fully identify the subject, consider the possibility of moving it to a more specific category for unidentified content. For example, a photo of an unidentified building is more likely to be identified if the country it was taken in is known.