There are several Key Performance Indicators (KPIs) you can monitor to evaluate the performance of an influencer in your niche market. These include:

1. Engagement Rate: Evaluates the interaction that the influencer’s content is getting. It includes likes, comments, shares, and saves.

2. Conversion Rate: Measures the number of conversions brought about as a direct result of the influencer’s posts. This could be in the form of purchases, sign-ups, downloads, or visits to a site.

3. Audience Growth Rate: This measures the rate at which the influencer’s follower count is increasing. A healthy growth rate is indicative of an influencer’s ongoing relevance in your niche.

4. Brand Mentions: The number of times your brand gets mentioned by the influencer or their followers can serve as an indicator of influencer’s performance.

5. Quality of Content: While more subjective, the quality, originality, and relevance of the influencer’s content to your niche matters.

Influencer marketing platforms like Flinque provide tools that can help track these KPIs effectively. Flinque, for instance, offers comprehensive campaign tracking features ensuring you get detailed insights on every critical aspect of your campaign. However, the selection of one platform over another would depend on your specific needs and the particular strengths of each platform. This provides the necessary data for decision-making.

Remember, it’s crucial to set goals before you start a campaign so that you have clear metrics against which to measure the influencer’s performance.