function domDetect(){
if(document.getElementsByTagName)
   {//DOM1 compliance ns6 and ie5 and soon opera 4+
   return 'dom';
   }
if (document.all)
   {//ie4  dom and opera spoofer
   return 'ie4';
   }
if (document.layers)
   {//ns4 dom
   return 'ns4';
   }
if(document.images)
 {//pre4 limited dom images support.
 return 'pre4';
 }
return 'noDom';
}


Then use the function elsewhere with either of the following:

if(domDetect() == 'pre4' )
 {
 //do something with a pre4
 alert(domDetect())
 }

if(domDetect() !== ( 'dom'  || 'ie4' || 'ns4'))
 {
 //do something else with other browsers
 alert(domDetect())
 }

When dom level2 support becomes more prevalent I would add.

if(document.Event)
   {//DOM2 compliance
   return 'dom2';
   }
