How can I tell the difference between unmarked sugar and stevia?
Don't throw it away.  Worst case you use up the mystery sweetener on yourself, and use a new pack of Stevia for your wife and anything you share. That's what I recommend if you're not convinced by my solution or don't have sensitive kitchen scales.

At room temperature, sucrose (normal sugar) is very soluble in water: about 200&nbsp;g of sugar will dissolve in 100&nbsp;g of water.

It's harder to get a figure for the solubility of Stevia as it's not a single compound but a mixture of related compounds.  However various patents claim "high solubility" stevia preparations of around 30&nbsp;g per 100&nbsp;g water, or around 1/7 as much.  

This is enough of a difference to test: If (with plenty of stirring but no heat) the mystery sweetener will dissolve in an equal mass (weight) of water, it's sugar. If not it's stevia.  This only works if you weigh it.  You can't do this by volume.

You can always compare to known sugar; a comparison to know stevia is less useful unless it's identical, and brands evolve. The conclusion holds for fructose too (that's even more soluble), but glucose, which you're unlikely to have as pure powder at home, needs slightly more than its own weight of water to dissolve it.

Pure stevia is much sweeter than sugar, and when concentrated may have an aftertaste. It's also sold blended. When blended with other sweeteners the solubility test probably still applies (certainly in the case of erythritol, find in Truvia). Sometimes stevia is bulked out to make it a more direct substitute for sugar. The bulking agents may well be rather soluble and a source of carbohydrates (maltodextrin is sometimes used).