How do i know if my download is done downloading

if you are using Safari as your web browser, you will see a blue progress bar in the upper right hand corner of the browser. Regardless of the browser, to verify, open finder, click the house icon on the left and double click downloads and see if the downloaded item is present.

During the download there should appear an icon on top of the Downloads folder in the Dock; it, too, should display a progress bar.

If you think the download is complete, open the Downloads folder in the Dock by single-clicking it. The download should appear; the manner of its appearance depends on how you have that folder set - as stack, as folder, etc. However that setting was made, the downloaded item itself should display a normal icon for the type of file it is - usually either a .dmg format or a .sit (or .sitx) format, sometimes a .zip format.

Note - downloads obtained through the Software Update feature of the OS (accessed via the Apple menu, via System Preferences, or run automatically) will not display in any of the preceeding manners. The Software Update screen will display full info about the download - time already spent, estimated time remaining, and size of the remainder of the download in progress.