How can I check if a player has already clicked a button before?

You could insert a BoolValue into the player (or the SurfaceGui but I'm not sure if this is a good idea) to check if the button has been clicked before. http://wiki.roblox.com/index.php?title=API:Class/BoolValue

The best way to do this would be to place the surfacegui into StarterGui. That way, you can track which player clicked it. An example:

LocalScript inside surfacegui:

script.Parent.Adornee = workspace.GuiPartExample

Then, to check if the player has already clicked it, you can insert a value into the player! Example inside a localscript:

local plr = game.Players.LocalPlayer
script.Parent.Button.MouseButton1Down:connect(function()
    if not plr:FindFirstChild('ClickedButton') then
-- do stuff
local value = Instance.new('BoolValue',plr)
value.Name = 'ClickedButton'
end)