How can I tell the difference between instant yeast and active dry yeast?
You can't really tell by looking, at least not without a known sample of the same brand.

The good news is that they are usually interchangeable 1:1. Make a recipe you know well. Does it rise as you expect? Or does it take more or less time? That will most likely give you your answer.

If the dough behaves as usual, it's a good bet that you have what you usually have to make that bread. If it rises fast, it's instant yeast. If it's slower, it's active dry. For the majority of loaves, the only difference you're likely to notice is the time it takes to rise and proof.

Joe says his answer, "When it doubt, treat it as active dry yeast, and proof it first", he's absolutely right if your recipe calls for proofing active dry yeast (not all do). If the proofing seems more vigorous than usual, that's a hint that it is instant. Get it into the dough right away, don't dilly dally while your yeast expending its life force.

King Arthur Flour gets into potential substitutions more detail. One thing they noted is worth mentioning here:


  One time when you might not want to use instant and active dry yeasts interchangeably is when you're baking bread in a bread machine. Since bread machines use a higher temperature to raise dough, substituting instant for active dry yeast 1:1 may cause bread to over-rise, then collapse. When baking in the bread machine, and substituting instant yeast for active dry, reduce the amount of instant yeast by 25%. 


So if you do both bread machine and traditional bread making, you might want to do a traditional loaf first using your unknown yeast.