The T/N and AD apply to all serial numbers. Elan didn't keep track of when they started deviating from the approved method of making the parts and substituted their own whimsical idea so the only way to find out is to open the wing up and look. By doing this you can tell if a particular DG300 has the spar problem or not but you can't tell by serial number.

The AD doesn't reduce the usability of the glider that much if so it probably isn't worthwhile cutting things apart for inspection to determine if a particular DG300 has a spar conforming to the original design or not. Might be worth looking around in there if you're already doing a serious wing repair as a result of an accident though.