Final answer:
The number of new clients is the most reliable indication of the value of external marketing efforts, as it directly represents the success of the campaigns in attracting new customers.
Step-by-step explanation:
To determine if the external marketing efforts of a practice are worth the investment, analyzing the number of new clients attracted after such campaigns is the most reliable indication. This is because the primary goal of marketing is to attract new clients to the business, thereby increasing the client base and potential revenue. Other metrics such as overtime pay, average client transaction, client compliance rate, or accounts receivable report, although important, do not directly measure the effectiveness of marketing efforts in acquiring new clientele.