That was the question every reporter including myself sought an answer to a few weeks ago in the flashpoint eastern Ukrainian town of Sloviansk, as clashes brewed between separatist and government forces.
Locals I spoke with at the time were in no doubt that gunmen from across the border were in their midst.
Yesterday that suspicion was borne out when Denis Pushilin, the leader of the self-proclaimed Donetsk People's Republic, who I interviewed in Donetsk City last month, admitted that some of his fighters killed recently had been "volunteers" from Russia.
"Those who are volunteers from Russia will be taken to Russia today," admitted Mr Pushilin, confirming that the bodies of separatists killed in the ongoing Ukrainian government offensive would be sent across the frontier.
After weeks of accusations from Kiev of Russian involvement in the uprising, it was an admission that might go a little way to bolstering the resolve of Ukrainian President-elect Petro Poroshenko, who has vowed to tackle the "bandits" in the east. Mr Poroshenko's government badly needs some kind of morale booster and shoring up right now, not least in a week that saw separatist militiamen briefly overrun Donetsk airport and yesterday shoot down a Ukrainian military helicopter near Sloviansk, killing 14 soldiers including a general.
According to Ukraine's outgoing president, Olexander Turchynov, the separatists used a Russian-made anti-aircraft system to down the aircraft and kill General Serhiy Kulchytskiy, head of combat and special training for Ukraine's National Guard.
Ukrainian interior minister Arsen Avakov, keen to keep the spectre of Russian involvement to the fore, insisted that weapons collected at the airport after the separatists were forced out by air strikes and a paratroop assault had been brought in from across the border. "These are not our weapons - they were brought from Russia. Serial numbers, year of production, specific models ... I am publishing this photograph as proof of the aggression of the Putin regime," Mr Avakov wrote on his Facebook page.
The biggest question now is how Russia will react to this latest Ukrainian offensive. Only a few days ago, Russian Foreign Minister Sergei Lavrov was insisting Moscow was "open to dialogue" with the new Kiev leadership, but that Ukraine's security operations against pro-Russian separatists must stop.
And there, of course, lies the rub. Should Kiev intensify the pressure against the separatists, as appears to be the case, then Moscow might be pushed to react. Should the Ukrainian government do nothing, then slowly but surely, as the past weeks have shown, the separatists will tighten their own grip in the east of the country.
Some political and intelligence analysts, however, are even of the belief that events on the ground have already moved beyond full control of the Kremlin. The genie of pro-Russian separatism is out of the bottle, goes the prevailing thinking in such quarters, and even President Vladimir Putin is not in a position to rein it back in.
While stirring things up might have been Moscow's intention from the outset, Russian intelligence officials and strategists would doubtless have been reluctant to relinquish total battlefield control to the myriad groups of separatist fighters that have sprung up and conducted operations off their own bat.
It would not be the first time that a large power initially involved in providing covert support saw their fledgling proxy "army" turn into a loose cannon throwing up a whole gamut of new political and diplomatic challenges that make the previous ones look tame.
This must have been food for thought yesterday as Mr Putin welcomed Kazakhstan and Belarus into a new Eurasian Economic Union built to rival the United States, EU and China. The very absence of Ukraine clearly undermines his dream of restoring Soviet glory days.
While Mr Putin denies he is trying to rebuild the USSR, he makes no secret that his dream is to reverse the consequences of its break-up by drawing former Soviet states closer together, and yesterday Ukraine's seat at the Eurasian table sat empty.
But if Moscow has tough tactical decisions to make, so too does the fledgling Kiev government. For his part, Mr Poroshenko has pledged that Ukraine will "not be turned into Somalia" and that he will put an end to the pro-Russian insurgency raging in the east of the country.
While he may yet end up winning the battle of Donetsk, he has a long way to go in winning hearts and minds in the region, and reconciling locals to Kiev's rule. The gloves are off in eastern Ukraine and days of fighting dirty lie ahead.