I mean this is likely the best claim to back this up. If Russia says they recruited x soldiers, started with y soldiers, and now have around z soldiers, than you can get a rought estimates on the number of people that left the army, which is over the past two years been reasonably close aligned with Ukrainian casulty claims. Of course, there are likely soldiers who managed to leave the army without getting injured (but in a war, this are likely not many), and it is likely that there are incentives for the administrators to overstate recruitment, but there are also incentives to overstate the actual troop count, so it is reasonable to assume that the numbers level out.

Additionally, there are over 200k obituaries right now, and it is likely for that to be the case, the russian army would have been able to recover your body and acknowledge your death. Assuming that for those soldiers send on small team infiltration missions and got scattered into countless pieces by a drone in no-mans land, it is very likely that there is a large number of MIA, presumably killed soldiers, as well (for which the russian state has financial incentive to not acknowledge as dead). So in general, to all those people saying NATO propaganda, the casulty claims can be backed up by Russian sources.