How to check if a textbox contains a number?

You can use string.match on the string to check if there's a digit/number. string.match returns something if a match is found. Otherwise, it returns nil.

if string.match(Textbox.Text, "%d") then -- %d matches a digit 
	-- TODO: Do something if there is a number in Textbox
end

For more info, read the wiki article for string functions and pattern matching.