There is no one definitive answer to this question, as it largely depends on the type of digital marketing service that you are investing in. However, some tips that may help you measure ROI include tracking monthly or yearly changes in website visits, leads generated through your campaigns and conversions achieved from those leads. Additionally, measuring how much money you have saved by reducing advertising costs or improving sales goals can also be helpful indicators of success. Ultimately, the best way to measure whether an investment in digital marketing services is worth it is to keep track of your results over time and make adjustments as needed.