Six signals that scope has already moved
By the time it is obvious, the free work is usually two days deep.
The signals
A new audience, a new format, a new decision owner, a reference to a competitor, a request to see a variation, and the phrase while you are in there. Six signals cover most of the drift that happens before anyone admits the scope changed.
Name it in the same email
The cheapest correction is a one-line reply: that is outside the current scope, here is what it costs. Sending it immediately costs seconds. Sending it next week costs a negotiation about work already done.
Small requests are the expensive ones
A request that takes twenty minutes still has a review cost, a context switch and a version control cost. Three of them in a week is half a day, which is why they belong on the change order like anything else.
The change log is the evidence
Record each order with a date and a price. At close, the log is what justifies the rate review and the revision cap, and it is the only document a client cannot argue with.