This will probably one of the shortest guides ever but I hope the information is useful for people.

It was prompted by a forum post and I will re-use the users error here.

The following message was being displayed;

Custom Assets: 488352822.Bus Underpass_Data: NetInfo missing [BrokenAssetException]

Details:
No details
BrokenAssetException: Broken assets:

Custom Assets: 488352822.Bus Underpass_Data: NetInfo missing

Now how do you know which of the many thousands of Workshop items do you have an issue with?

You could try searching for Underpass and Bus in the workshop or you could make a note of that number, 488352822.

This is the ID of the item in the Steam workshop.

If you take that ID and add it to the URL;

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=

It will take you directly you directly to the right page, e.g.

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=488352822

Hopefully this will help some people

A lot of people ask this and this is the information from Paradox

How to retrieve the Log file:

• On Windows, the log file is found in the Steam game install folder under Cities_Data/output_log.txt (to open the game install folder, right click your game in the Steam library, select Properties, go to the Local Files tab and select Browse local files)
  
• On Linux, the log is found at ~/.config/unity3d/Colossal Order/Cities: Skylines/Player.log
  
• On OSX, the log is found at ~/Library/Logs/Unity/Player.log

You should then Upload the file to a sharing site like Pastebin (http://pastebin.com/) and link to it in the forum post where you ask someone to explain the log file to you :)

I've just discovered a guide that goes into much more depth than I've gone into here. So for users who want more detailed information about reading their Logs I suggest they visit this Guide;

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=463645931