ump45...
...if you examine the area of the slide just behind the breech block [towards you as you hold the pistol naturally], you'll see one of two methods for securing it:
1. By a double roll pin [one inside the other]; or...
2. A solid pin.
The latter is for the milled stainless slide.
No milled stainless slides are mfd. in 'blued' version.
In addition, there are differences in the slide serrations meant to aid in charging the pistol.
Full-length serrations on the back of the slide almost always mean that it's a stamped-steel slide vs. the milled stainless.
Hope this helps...