I think concealability is a result of the cut of the fabric more than anything, based on the various attires that have the modifier and those that don't.
Armanté Suit/Dress; No modifier
Mortimer of London Berwick Suit/Dress; -2 modifier
Mortimer of London Crimson Sky Suit; -2 modifier
Mortimer of London Summit Suit/Dress; No modifier
Mortimer of London Greatcoat/Ulysses/Argentum; -3 modifier
Vashon Island Ace of Cups/Swords/Wands/Coins; No modifier
Vashon Island Ace of Spades/Clubs/Hearts; Holster (not specified if it's quickdraw, concealable, or concealable quickdraw)
Vashon Island Ace of Diamonds; +2 modifier (probably supposed to be a -2)
Vashon Island Steampunk; No modifier
Vashon Island Synergist Business Line; Concealed Holster (so only -2 for pistol sized items)
Vashon Island Synergist Business Line Longcoat; No modifier
Vashon Island Sleeping Tiger; Holster (not specified if it's quickdraw, concealable, or concealable quickdraw)
Zoé Executive Suite; No modifier
Zoé Heritage; No modifier
Zoé Nightshade/Moonsilver; No modifier
Zoé Second Skin; No modifier
Ares Victory Globetrotter Jacket; Holster (not specified if it's quickdraw, concealable, or concealable quickdraw)
Ares Victory Globetrotter Vest; No modifier
Ares Victory Globetrotter Clothing; No modifier
Ares Victory Wild Hunt; Holster (not specified if it's quickdraw, concealable, or concealable quickdraw)
Ares Victory Industrious; No modifier
Ares Victory Big Game Hunter; Holster (not specified if it's quickdraw, concealable, or concealable quickdraw)
Ares Victory Rapid Transit; No modifier
The interesting part is that there are three different ways of writing concealability modifiers in Run & Gun;
-2 modifier to Concealability
Concealability +2
Concealability (-6)
The first one refers to longcoats and suits, the second is likely a misprint unless the Ace of Diamons is supposed to make it harder to smuggle items, and the last one refers to the difficulty of spotting Form Fitting Body Armor underneath regular clothing.
There's also no rhyme or reason I can find as to which items provide a modifier and are also custom fit; I would have presumed that an item that is custom fit for you (i.e. tailored to fit your frame perfectly) would not have a lot of room to conceal items, but apparently this is not the case.
So, we're left with whatever's in the book, for better or worse. Houseruling it is a pretty simple solution if you ask me.