How to check if a player has a tool equipped in their hand?

When you equip a tool, it ends up inside the character. When you want to check if a tool is being equipped, call this function:

local character = game.Players.LocalPlayer.Character

function toolEquipped(character)
    for _, child in pairs(character:GetChildren()) do 
            if child:IsA("Tool") then 
            return true 
            end
    end
    return false
end

if(toolEquipped(character))then
    print("There is a tool equipped")
else
    print("there is not a tool equipped")
end

If you want t know if a certain tool is equipped, then check the Tool's equipped property:

Tool.Equipped:Connect(function(mouse)
    print("Tool was equipped")
end)