In order to find out which model Roomba you have, turn the machine upside down and look for a sticker that contains the serial and model number.
There are basically 2 different series of Roomba's, the newer 500 series, and the older 4000 series Discovery series. These models usually had multiple buttons on the top, unlike the newer 500 series which have just one.
The older series models included a white, red, green, and yellow colored machines, where as the newer 500 series models have a more sleek design, usually with just a single button in the center. The newer 500 series have colors of black, and silver with different combinations of the two.
Here's a link to pictures of all of the different Roomba series ever made, and the models that they contained:
Also, in terms of the circling problem your Roomba is having, here's a very helpful link that will show you how to fix the "Roomba Circle Dance" as it's been named, haha dancing robotic vacuum cleaners, I just got a mental picture, lol.
I really hope this was helpful to you. Please feel free to ask any follow up questions you may have, I'm always more then happy to answer any and all of your questions if this was not able to fix your problem. Please check out our website for all of your Roomba accessories:
Thanks so much. If this was helpful, please be so kind and rate my repair helpful, or leave feedback, thanks!