You will only see a red bar or green bar above clips that need to be rendered.
Video that is being edited natively will have neither above them.
This ARTICLE goes into a bit more detail on what Rendering is, and what it does. Note: it was written for both PrE and PrPro, and as of PrPro CS5, there is also a yellow line, but you will not see that in PrE, so just ignore any reference to a yellow line.
Good luck, and hope that this helps,
Thank you - most helpful!