Wow, do not throw it away, especially if you have a scale or yeast handy. There are 2 easy methods to determine whether the unknown sweetener is sugar or not.
Try fermenting unknown sweetener with yeast
With the exception of lactose, yeast can feed on all "real" sugars, or at least the ones you'd normally keep in your kitchen. On the other hand, in all of my testing, I was unable to get any "artificial" sweeteners to be fermented by yeast. Those tests included Splenda (maltodextrin + Sucralose), erythritol, and even allulose (psicose). The same would likely be true for Stevia granular sweetener, as it's a mix of maltodextrin + stevia.
Use weight and taste to determine sweetener type
First of all, pure stevia powder that isn't cut with anything would be incredibly sweet (with possibly a bitter aftertaste): the sweetness is much more concentrated than sugar. Generally, pure sugar substitutes like stevia extract, sucralose, etc. are usually "cut" or bulked with less sweet ingredients like maltodextrin (hardly sweet at all) or erythritol (75% as sweet as sugar).
1 cup of sugar weighs around 200 g.
1 cup of Truvia (erythritol and stevia extract) should weigh around 224 g.*
1 cup of Swerve (erythritol, oligosaccharides and natural flavors) should weigh around 204 g.*
1 cup of Splenda (maltodextrin + sucralose) should weigh around 27 g (yes, it's that light).
1 cup of maltodextrin + stevia extract should also weigh around 27 g.
If she's ever used this sweetener for baking, then it's unlikely that it's Equal (aspartame), as that loses its sweetness when subjected to heat.
* Note that any sweetener that contains erythritol will be easily distinguished by the extreme cooling sensation it has on the tongue compared to other sweeteners. With Swerve in particular, I've found the that the sweetness sensation seems to linger for minutes after I've tasted it (provided I don't eat anything else). I don't get the same experience with Splenda. Sorry, I don't know how this compares with sugar (sucrose), as it's probably been 7 years or so since I've tasted it.