A slack line bite shows in exactly the same way when you are using a plain lead. The only difference being that your rod may not have the initial curve as it does with a grip lead. The procedure to follow is the same as above.
Question: How do I know it’s a fish that has produced the slack line and not the tide?
Answer: If the tide was responsible for picking up your lead then your line would travel with the tide. Your line would remain tight even if washed up further down the beach. Breaking loose doesn’t produce slack line.