Answer: When Russia denies it and Nato calls it an "incursion".
For months now, speculation over the extent of Russia's military presence in eastern Ukraine has resulted in a war of words that yesterday culminated in Ukrainian President Petro Poroshenko saying Russian troops have "been brought into Ukraine".
So significant is this latest development that the Ukrainian leader instantly called an emergency meeting of the country's National Security and Defence Council.
President Poroshenko's assertion was almost immediately endorsed by a Nato military officer who said the alliance believed well in excess of 1,000 Russian troops are now operating inside Ukraine.
Admittedly this does not constitute a full-scale invasion - yet - but at the very least it gives greater credence to Western officials' growing belief that Russia's aim is to carve out a land bridge to the Crimean peninsula, which it annexed earlier this year.
Even as Russian leaders were meeting a few days ago with Ukrainian and EU counterparts in Minsk to discuss the ongoing situation in eastern Ukraine, Moscow has been quietly shaping the battlefield for maximum leverage in any negotiations.
Since its July 4 offensive began, Ukrainian forces have made considerable frontline progress but what has become equally apparent is Moscow's mobilisation of its military and intelligence assets to turn this around. It is one thing to supply weaponry, another to provide the kind of support that has made the pro-Russian separatist forces much more effective on the ground.
Viewed from a Western perspective, at face value Nato would appear thus far to have been fairly low-key in its response. This, however, could not be further from the truth.
Indeed, the alliance has already pledged to deploy troops to new bases in Eastern Europe.
As part of that process, we will see soldiers serving under the Nato banner being sent to a former Soviet bloc nation for the first time ever.
Ukraine itself, of course, does not belong to Nato, so the alliance is not obligated by treaty to deploy ground troops or air support. And while the alliance could supply Kiev with weapons, it would be up to the Ukrainians themselves to do the fighting.
As Nato sees it, the so called "Putin doctrine" that believes Russia has the right to act to protect Russian-speakers, no matter where they are, puts Nato nations such as Estonia, Latvia, and Poland at risk.
However, back on the battlefields of eastern Ukraine right now, Ukrainian forces have been up against it. Not only have they suffered significant losses over the last few months since separatist forces surrounded the spearhead of the Ukrainian offensive near the Russian border, but tremendous firepower delivered by Russian rocket and tube artillery in this frontier region has led to significant attrition.
It is estimated that more than 2,000 Ukrainian forces are located inside the area encircled by separatist fighters.
While not yet on the point of collapse, they remain cut off from supplies and reinforcements, and it not inconceivable that these Ukrainian troops may have to fight their way out on their own if Kiev is unable to mobilise sufficient resources and reinforcements to break the encirclement they face.
No doubt like many of the readers of this column, I have been amazed by the apparent lack of urgency shown by Europe and the international community in addressing the crisis.
Admittedly, there is no shortage of dangerous flashpoints around the globe right now. But what has been brewing in eastern Ukraine somehow seems to have gone unchecked, and this too after the shooting down of Malaysia Airlines flight MH17 by groundfire from separatist-held territory that killed all 298 passengers on board. Strange how this and the outrage it generated seems to have dropped off the political radar.
Yesterday, however, Europe seemed top be waking up to the unpalatable fact that Moscow has no intention of backing off and indeed looks to be upping the military ante. While no western official has yet got round to calling Russia's military presence in eastern Ukraine an invasion, Carl Bildt, Swedish foreign minister, at least admitted there was only one word now to describe the fighting. That word, said Mr Bildt, is obviously "war."