Using bukkit or other servers that can load plug-ins can easily determine whether there is a choice of "Yes" or "No":

 

if( event.getStatus() != ....SUCCESSFULLY_LOADED ){
          //The player failed to load the texture pack correctly
}

 

But now it’s Forge mod! What should i do? please help me!

 

19 minutes ago, diesieben07 said:

When the user accepts or declines the resource pack, a CResourcePackStatusPacket is sent back. Vanilla just ignores this packet and so does Forge. Apparently Bukkit adds some functionality to this response packet.

You can make a pull request to Forge to add an event here or make a feature request on Github.

In both cases please outline your use-case for this.

Can you tell me how to do it? Is there any sample code?