How do you detect key presses?

Use the KeyDown Event. You can define WASD using the second parameter.

Syntax: KeyDown (String, key).

Inside a script inside a hopperbin:

function onSelected(mouse)
    function onKeyDown(key)
        print(key) -- That's the key you pressed.
    end
    mouse.KeyDown:connect(onKeyDown)
end
script.Parent.Selected:connect(onSelected)

Hope that helps!

If you were trying to make a custom camera or a 'Probe' as some would say, you could always use Workspace.CurrentCamera.Focus or Workspace.CurrentCamera.CoordinateFrame (focus for in game coordinate frame for nil) But in this case, the one I am giving you is detecting which key is being pressed in the movement category, and if it matches it then it prints which way your going.

s = game.Players.LocalPlayer
m = s:GetMouse()
m.KeyDown:connect(function(a)
if a:lower() == "a" then
print'left'
elseif a:lower() == "s" then
print'down' 
elseif a:lower() == "w" then
print' up' 
elseif a:lower() == "d" then
print'right' 
end
end)