In a radio interview today, People’s Deputy Boris Tarasuk (of the Ukrainian Parliament) said that Russian troops had been in Ukraine and that they might now still be, he didn't know. He said they were dressed differently and their weaponry was different from what the Ukrainian military uses.
The accent of Ukrainian Russian speakers is different than the Russian accent of Russians. Ukrainians can easily tell the difference.
UPDATE: The deputy said they even knew the address where these troops bivouacked. It was at a place on Shovkovitsna Street.