they would know because there is very tight security against exactly that.. because defection of pilots is always a particularly outsized risk for them.. they are probably better prepared to shoot down their own aircraft than the opposition.. same with their front line troops..

it's just life there.. the generals sell off ammo and fuel to pay for yachts and gambling debts.. their hotshot pilots all want a fat paycheck for their jet, and to go live in Serbia..

I figure they may be able to compare flight plans with actual course and tell when a pilot is diverging from the planned route. Based on how they are diverging, it may suggest that they are trying to book it to Ukraine? Not sure if this is actually possible, but it's the best way I can imagine them being able to tell, other than the pilot just disobeying immediate orders and heading for the border, which would be pretty obvious.