Basically, and this may be not-so-easy, the way I can tell is at the bottom of the EQUI_CastSpellWnd.xml file (your spellbar) there is something that looks kinda similar to this I hope (taken from default UI)
Code:
<Pieces>CSPW_Spell0</Pieces>
<Pieces>CSPW_Spell1</Pieces>
<Pieces>CSPW_Spell2</Pieces>
<Pieces>CSPW_Spell3</Pieces>
<Pieces>CSPW_Spell4</Pieces>
<Pieces>CSPW_Spell5</Pieces>
<Pieces>CSPW_Spell6</Pieces>
<Pieces>CSPW_Spell7</Pieces>
<Pieces>CSPW_Spell8</Pieces>
<Pieces>CSPW_SpellBook</Pieces>
Count 'em up, 0 1 2 3 4 5 6 7 8 there are nine. So this is a method you can use, if the screenshot of a particular spell bar mod doesn't show if it has a ninth slot and the description doesn't say either. The CSPW may or may not be present; I do not know for each particular UI what they name their pieces.
Be careful, if you find a spellbar that is missing the ninth slot, just adding a line here won't add a ninth slot, it will make your UI angry.
Edit: Oh yeah, here is a link to
NecroSkullSpellbar and I couldn't find anything called skgems spellbar. The NecroSkullSpellbar does
not have a ninth slot.