The canonical experiment is to measure the optical activity (rotation of plane polarized light) by a solution of the substance. Dextrose, unsurprisingly, is dextrorotatory (rotates light to the "right"). l-glucose is levorotatory (rotates to the "left"). Two things to keep in mind: There are different d and l nomenclatures. A compound labeled "D-" is not always dextrorotatory.) Also, optical activity is only definitive if you've already established the compound is d-/l-glucose. If it's fructose ("levulose") or some other carbohydrate, the normally occuring isomer could be the levorotatry one.
Given that you likely don't have a polarimeter handy, another easy experiment you can do is to add some yeast to a weak solution of the sugar. Yeast can readily use d-glucose, but should have a difficult time fermenting l-glucose. The reason for this is that the enzymes which catalyze the utilization of glucose are chiral, and will only match the typical d- configuration. l-glucose won't fit, like a left foot doesn't fit well into a right shoe.
All that said, it's so exceedingly unlikely that you were sold l-glucose that it boggles the mind. First off, there's the labeling concern. "Dextrose" specifically means the d- isomer, so anyone selling l-glucose as dextrose is committing fraud. Secondly, the price differential is astronomical. Spot checking, from a chemical supplier l-glucose costs over \$50 per gram (so ~\$400 per teaspoon). In comparison, d-glucose of a similar grade from the same supplier is around \$0.04 per gram (so \$0.30 per teaspoon). With over a thousand-fold price differential, someone along the way would recognize if they were accidentally selling l-glucose.