3 main issues:
Are well known local informants able to confirm information?
Check the picture elements (places, decor, clothes...).
Does the image shows really the place the source is talking about?
Example of questions you can ask the source according to Alex Murray, journalist at the BBC, UGC Hub: “You ask the same questions that you would ask anyone talking the story : what’s the weather like today where you are? Something you can check out […] where are you standing? What can you see from there? What are the streets' names? […]”(Alex Murray, journalists, UGC Hub, BBC, see the reference below).
The verification is based on "a combination of several things: interview someone who tells the story, check the history in official documents if they exist (court documents, police data, figures, etc.) and get details for institutions, press releases or press conferences. " (Agence France Presse Newsletter) (cf. bonus material).