How do you check that a ModuleScript is being called locally or serverside?

I am doing this for security reasons with my module I'm making.

RunService

You can do this by using the IsClient and IsServer functions of the RunService, for example:

local rs = game:GetService("RunService")

if rs:IsServer() then
	print("server")
else
	print("client")
end

Hopefully this helped!