How can startups measure the effectiveness of their marketing strategies?

Really depends on the goal of your company/team! For most companies though it's a mix of getting new users, retention and organic growth

Google analytics

Customer Acquisition Cost (CAC), Return on Investment (ROI), Email Open and Click-through Rates, Organic vs. Paid Traffic, Conversion Rate, and Net Promoter Score (NPS). Dependent on the magnitude of users, A/B testing is vital.

By monitoring ROI, startups can determine which marketing strategies are most cost-effective and contribute the most to the company's growth, enabling them to make data-driven decisions and optimize their marketing investments.