In general, a character wearing appropriate clothes can always hide an item, and a character without appropriate clothes cannot. You can use Investigation or Perception to spot a hidden weapon, with a difficulty of the bearer's Deceit or Charisma. Metal detectors and such requires FixIt to operate rather than Investigation.
Items have concealment ratings, which determine how large and/or bulky they are, and hence what kind of clothing they can be hidden under. Concealment ratings have the following broad categories:
Negligible
This item can be hidden when you are in a swimsuit or, and is small enough to hide in your mouth or up your ass. This is the only type of gear that can be hidden from a strip search. No more than ten centimeters long and two centimeters in diameter.
Small enough to hide in a pants pocket, an evening dress, or under an evening jacket. These also fit in a purse. A pocket-sized object can be smuggled into a ballroom and might pass a pat-down search. About fifteen centimeters long, two centimeters wide and ten centimeters high. It needs to be without protrusions.
Jacket
Can be concealed under normal outerwear or bulky indoor clothing. Big guys in form-fitted suits can just barely conceal it. Any pat-down will discover the gear. About twenty-five centimeters long, three centimeters wide and fifteen centimeters high.
Trenchcoat
Now we are talking big stuff. It takes a long coat to conceal one of these, and there is no chance of passing even a cursory close inspection. About fifty centimeters long, ten centimeters wide and thirty centimeters high.
Unconcealable
These are just to big to carry around hidden, though you cold use a golf-bag or ski-bag, depending on the gear.
Support
Support weapons normally cannot be used from the hip - they must be fired from a tripod, or by prone characters using a bipod. This means you cannot move and fire these weapons.
Slick
Some items are not concealed by virtue of being small, but rather by seeming harmless. A slick item will not ordinarily attract attention or have any obvious harmful use.