Shattered Kingdoms

I was wondering how some people to seem to know that the "Great rune of XXX" is 5 instead of 4 on stock items within the game. Is there some way of telling this for sure?

If it is a stat increase, in most cases it is easy to find out. If it is anything else it could only be conveyed by a builder.

Often it is just guessing. It is a safe bet that the Ephialtis armor has 5 MP on each piece, for example. The builders are allowed to have something start with 5 runes max, and it is clear that they would want that armor to be as good as possible. Another example would be that Karnak's pike and the crackling energy dagger are probably +5 to damage. This is essentially making an informed guess, though.

The only way to know for sure would be for an IMM to tell you. Some players have had that kind of contact with IMMs over the years, and could have knowledge about certain items.