<p>I have a Prusa i3 MK3 or maybe it was upgraded to a i3 MK3S.</p>
<p>How can I figure out?</p>
<p>The <a href="https://shop.prusa3d.com/en/original-prusa-i3-mk3s/1390-original-prusa-i3-mk3-to-mk3s-upgrade-kit.html" rel="nofollow noreferrer">description of the upgrade kit</a> talks about</p>
<ul>
<li>the SuperPINDA (how is it different from the old one?)</li>
<li>a number of small changes (which?)</li>
<li>improved plastic parts (which parts, how are they different?)</li>
<li>metal clips (where to look for them?)</li>
<li>a number of minor changes to the extruder plastic parts (which ones, before and after?)</li>
</ul>
<p>I'd like to figure that out without taking the printer apart.</p>

The MK3 has 4 pins on the filament sensor, the MK3S has only 3 pins. While you need to take the extruder apart to see that, you can also have a look at the cable instead:
Matching the sensor, the MK3 has a 4 strand wire including blue and the MK3S has a 3 strand cable without blue.
Looking under the heatbed, you'll find the MK3 bearings are fixed with U-bolts while the MK3S has broader bearing clips.