How can I know which brand hard drive is in my iMac?

Open up About This Mac off of the Apple menu icon. Then click on More Info ..., now click System Report ..., then click on Storage under the hardware twisty listing.

At this point you should see the details of your HD. Here you'll want to copy under Physical Drive section the Media name entry without the word Media Now open Safari and past that into the browser it should come back with the a few entries which identify which drive you have and the manufacture.

I assume you are thinking on upgrading your HD or its starting to fail? If so lets talk about that before you go forward here. Tell us whats happening we maybe able to save things or guide you to the best choice of HD to replace your current drive.