added a convenience method.

core/src/main/java/hudson/model/Computer.java

@@ -695,6 +695,16 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
         return true;
     }
 
+    /**
+     * Returns true if this computer has some idle executors that can take more workload.
+     */
+    public final boolean isPartiallyIdle() {
+        for (Executor e : executors)
+            if(e.isIdle())
+                return true;
+        return false;
+    }
+
     /**
      * Returns the time when this computer last became idle.
      *