IIS 8 - 500 server error logs

You can enable "trace logging" on IIS Manager. It fill create bunch of XML files you can easily investigate using IE installed on the server. (other browsers may fail to process xslt style files)

From Technet:

1. Open IIS Manager and navigate to the level you want to manage. For information about opening IIS Manager, see IIS 7.0: Open IIS Manager. For information about navigating to locations in the UI, see IIS 7.0: Navigation in IIS Manager.
2. In the Connections pane, click Sites.
3. In Features View, select the site for which you want to enable trace logging.
4. In the Actions pane, under ``Configure, click Failed Request Tracing.
5. In the Edit Web Site Failed Request Tracing Settings dialog box, select Enable to enable logging for this site.
6. In the Directory text box, type the path where you want to store the log files or click the browse button (...) to find a location on the computer. The default is %SystemDrive%\inetpub\logs\FailedReqLogFiles.
7. In the Maximum number of trace files text box, type the maximum number of trace log files that you want to keep, and then click OK.

PS: don't bother IIS 7.0, almost same as IIS 8.x

This should be a comment but comments don't let me add an image.

This is a screenshot of Matt Browne's comment. Upvote his comment if you find this helpful.

If you don't see the "Failed Request Tracing Rules" button, then you need to enable "Tracing" tracing in Server roles.

Here it is:

There is also this configuration that needs to be set:

I had to restart my machine in order for the server configuration to be recognized in IIS.