Wikipedia has articles over the most of the sources worth to consider. You can check such article for who owns the source, and what is the reputation, history of it. If it is a web based source, simply put the domain name into Wikipedia search box. No Wikipedia article usually means it is a little known source, these are generally unreliable. But even more unreliable are state owned media outlets. For the sides at war, everything they publish about the enemy losses is void. Use domain name (ru? ua?) for the quick pre-check.
For community sites and social media posts, always check the sources, if they exist, do they write that is claimed and how reputable do they are. If the source is in the language you do not read well, use Google translate.
For Wikipedia, cross check articles in more than one language. This often means different list of sources.
For various "proving photos", demand the source and do some analysis. If very low resolution, why? Has it been edited? Here are some hints what to look at. These proofs are not as good as they look: lots of posted material may be forged or irrelevant.
Always search for the historical context of the events described: what happened before? After? Biased sources often omit even widely known context.
Ask yourself, how do the source knows the information and what is they motivation for sharing it.
Some propaganda may still be so blatant that there is no way to negate it by analysis of the posting itself. Then only reputation of the source can help: does it have the history of publishing nonsense and does it have any history at all.