From the OP's description of events, it's a little hard to tell if the three "cauliflower" plants put into the ground were replaced by three Brussels Sprouts plants, or grew into 3 Brussels Sprouts plants. If the former, most likely, I'd say someone is playing a practical joke on you, albeit a tasty one. If the latter, well, mislabeling happens all the time in garden centers, and the transplants sold to you as cauliflower were obviously broccoli. All of the brassicas look superficially similar as young transplants, although with a little practice it's generally fairly easy to ID them by sight. But, even a label isn't a guarantee, as many of us have learned over the years.

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.

We don't worry much about frost down here. But while we have had a moderate summer, the current conditions are the hottest we have had. Air temps of 100 degrees, powder dry, and in the after noon soil temps will burn your bare hand. Established summer plants like okra are laying down in the afternoon even with irrigation. Plant a nursery brassica without shade and it will be dry enough to roll up and smoke the first evening. Yolos; nursery plants are especially tender, so I would recomend that you place them to receive sun light in the morning and afternoon shade to help harden them off. Hopefully we will get some rain to cool the soil. I grow my own plants so they are quite tough, but not tough enough to take present conditions in the open field.

Hi Everyone, I was growing brussels sprouts for my vege garden and starting the hardening off process. Accidentally, I left them outside the other night and I think it got way too cold and I lost all my brussels spouts seedlings!!! My broccoli and parsley plants made it, but not the brussels sprouts. :( Does anyone have any they can spare? Judy