How to check if table index is this string?

This should return the spot in the table whatever you specify is in, and nil if none:

function getIndex(table, item)
	for i, v in pairs (table) do
		if v == item then
			return i
		end
	end
end

EDIT: I didn't include an example..

if getIndex(InvTable, "Iron Ore") ~= nil then
	-- Do stuff
else
	-- Do other stuff?

Hope this helped.

EDIT 2: The getIndex function returns which spot it's in, not a boolean value. So, it should look like this:

if player.Smelting.Value == true then
    if getIndex(invtab, "Copper Ore") ~= nil then 
         local copper = getIndex(invtab, "Copper Ore")      
            table.remove(invtab, copper)
            CheckForEmptyBox()
            local numb = selectedbox.Numb
            table.insert(invtab, numb.Value, "Copper Ingot")
            player.Smelting.Value = false
            player.Smeltwhat.Value = ""
    else print("Nope")
    end