They're on the Internet. 
How do we know the stories reported are true?
Simple, the msm says dance to whatever naritive they report. If you don’t, you are a terrorist, extremist or what ever *ist they lable you with. In this case it’s putinist. It’s not that complicated.
Because if the stories were NOT true, Russia would control Kiev/Ukraine and Zalensky would be dead
They're on the Internet.
Because they are not in Kyiv. Man, you PutlerLovers\Reptilian are taking over the world conspiracy theory dummies are dense.
Which stories ?
That question seems to appear whenever the war in Ukraine is the topic but not on other threads about things like the situation on the US/Mexico border. You can use the same techniques you would use to figure out if there really is a flood of illegal aliens across the southern border to try to understand what is happening in Ukraine.
When you see videos of illegal immigrants crossing the RIo Grande do you think that it really is happening, or you can't know for sure because the video might be fake and the people might just be actors hired by somebody?
There is more objective data available about the war in Ukraine than for any other conflict in history. You can watch videos of combat, and if you want you can look at thousands of photographs and videos of the consequences. In some cases you can watch video taken during combat, and see combatants or civilians killed as you watch.
You can look at collections of data, like thousands of images of destroyed tanks, trucks, howitzers, and other vehicles from both sides. You can look at satellite images, and see the same smoke you see in images taken from the ground or from attacking drones.
You can even see the density of people using Russian cell phones that are roaming on Ukrainian cell phone networks. It turns out that even in war young people from Russia bring their cell phones and use them.
There is plenty of data out there to look at if you want to know what is real and what is not.