I don't know about how to distinguish between the two, but I do know that the two have more or less the same needs. You say the leaf tastes like a broccoli leaf. In my experience, all crucifer leaves taste like cabbage. I don't think that taking a bite will help. In any case, if your plants have the same colour and the same size, then they are most likely both the same plant. I seriously don't think you can tell the two apart with young plants. Both broccoli and brussels sprouts have the same plant structure, and although I have never grown brussels sprouts, I hear it also has a small broccoli flower at the top. You may never find out how to differentiate them, but in case you only need to know because you would like to handle your plants correctly, rest assured that both require about the same conditions. They are also of comparable size (well, the sprouts may be taller) and therefore need about the same spacing as well.

I'm having the same debate with myself about some sprouting broccoli from spring. Now that the fall broccoli seedlings are up and growing, keeping the old plants seems like a bad idea. They are too big for row covers, so I'd have to spray them with Bt regularly and I don't think they will be worth the hassle. Around here spring-planted brussels sprouts always blast (produce loose, leafy sprouts) because they mature in hot weather. If your plants have held their fire this long, they will probably start budding soon. I suggest staking the plants, even if they seem sturdy now. In my experience, upright plants produce much better than those than tilt.

alamo5000, I wanted to respond earlier with a photo but kept forgetting to take it. We are in the same climate zone. Broccoli, cauliflower, and brussels sprouts do wonderful as a fall/winter vegetalbe in our neck of the woods. I usually set out about 150 to 160 plants each year. They tend to do fine as long as the temperatures stay mid 20's or above. They do get a bit stressed in the lower 20's. And, when the temperature stays below 20 for long periods, there will be plants losses. Anyway, here is a photo. Plants on the back row were planted early September. They are just beginning to show heads. Plants under the date were planted the mid to late October. These late plantings are past our recommended planting window.

"They can't be spring planted for fall harvest, in late summer the weather needs to cooperate to get them making sprouts before the cold stops them, and winter needs to be on the mild side for them to make it through to spring." Wow, it's hard to believe your climate is so radically different from mine that growing brussels sprouts is impossible. I always get plants out in spring for fall and early winter harvest, and they grow nicely through summer, providing sprouts as early as late summer (even the past very hot and dry summer here (record number of days above 90F) didn't affect the crop). I picked a lot of sprouts just before our late December snows and cold snap (down to 10F this morning), and there may or may not be much that makes it through to the next thaw. Still, the 2012-13 harvest was very good, even if my plants didn't look as orderly and nice as the catalog photos. Chiltern Seeds has a couple of new brussels sprouts varieties listed in 2013, one of which (Montgomery) I'll be trying.