I don't think there are differnt methods of determining what is a off-limits target, but I am not a military expert by any stretch of the imagination.
There are videos in Ukranian by Ukranians telling the Ukranian forces to move their cannons because they are close to schools and thus making civilians targets. I put weight on such videos because the Twitter user who posted them had their account deleted and the next person (that I know of) who posted them had the tweets deleted.
The alleged theater bombing in Mariupol has no video, no report of casualties, and no independent corroboration. I'm not taking the word of the Azov Battalion (who made the claim) if the Russians targeted a theater full of kids. The Ukranians are doing really well militarily in 2 of 3 major conflict areas, but in the East (Mariupol) they are losing. That the claim of the attack on the theater came on the day Zelenskyy addressed the US congress is (for me) a bit too on the nose.
Where am I going with these examples? I look at the last US / Russia proxy war in Syria, where the last thing Assad and the Russians wanted was direct US action, and that is exactly what the ISIS-lead rebel forces wanted. So what did we get? Video after video of alleged atrocities by groups like The White Helmets (an ISIS-aligned group) claiming Syrian atrocities (the two largest fake stories are the 2013 and 2018 gas attacks).
The claims that Ukranian forces are using human shields and shooting people trying to leave fighting areas are as many or more than those of Russians targeting civilian targets. The truth may never be known.
More shortly put, there is propaganda on all sides. Russian, Ukranian, US. The line between "selective reporting" and "propaganda" is subjective.
Regarding Mai Lai, the average US citizen may have been and is horrified by such war crimes, but at the time and many years after, the US military did everything it could to cover them up. Colin Powell was directly involved in the coverup of Mai Lai. Yes, that was over 50 years ago, but Julian Assange is rotting in Belmarsh for publishing video of US war crimes in the Mid-East. Chelsea Manning was tortured for leaking that video.
The US tortured Iraqis and made "funny" pictures. Guantanamo Bay is still open. George Bush, Dick Cheney and co. lead from the top that torture is OK if we do it. We - the US - are no better than the Ukranians or the Russians.
Every year there is a resolution condemning Naziism at the UN. Only two countries vote against it: The US and Ukraine. That says something about priorities.
The US talks about war crimes, but we are NOT a member of the ICC. Actually, there is a law from 2002 that the US will attack the Netherlands (The Hague) to "rescue" an American brought to the ICC on war crimes charges.
We are not better, only more powerful.
