How can I see my .NET error from my client ?

You can enable full error reporting in your asp.net application by changing or adding the following line to your web.config file. It is under system.web in your web.config file.

Set customErrors mode="On" or "RemoteOnly" to enable custom error messages, "Off" to disable.
Add tags for each of the errors you want to handle.
"On" Always display custom (friendly) messages.
"Off" Always display detailed ASP.NET error information.
"RemoteOnly" Display custom (friendly) messages only to users not running on the local Web server. This setting is recommended for security purposes, so that you do not display application detail information to remote clients.