So my brother found a open bag of a LEGO Chima kit. It had some of the pieces in it. I want to replace the missing pieces but I don't have the box. I tried looking for unique pieces but it didn't seem to have any. I also don't have the instructions.
Choose a brick, the most distinctive the better.
Look for the part number. These are on the underside of plates, or inside bricks. They can be microscopic but every piece had one.
Go to peeron.com. Enter the part number into the search box. A list of all the sets containing that part number should, with luck, come up.
If you need to, repeat with another brick. Because you know it's a Chima set you should be able to deduce the set, set number and parts list fairly quickly. I've done this many times.
Do you have any of the minifigures from the set? That would be the easiest way to identify the set. Other than that, your only option is to look for the most unique looking part and see which sets it appeared in. I know you mentioned none of the parts appear to be unique, but sometimes just a color variation can narrow it down. Anything like wheels, slopes, larger plates can help, as they appear in less sets. Also, you may want to post a picture here of the parts assortment you have, and see if someone can identify the set for you.