If you go in via World Drive, it is extremely noticeable (below is an image from street view - Google Maps):
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Note that you would enter through the lanes to the right, but left of the last lane that all the buses use.
If you come the back way, avoiding the normal WDW places such as Disney Springs, you will still have some signs, but not as many. When you arrive at Fort Wilderness, the sign does indicate "Disney's Fort Wilderness ...", but the "Disney" is small. Now, once at Fort Wilderness, there are many subtle points to being at WDW.
Now, after writing all of that, I just realized the biggest thing that screams WDW is all of the WDW buses driving around on-site.