How do you measure SEO success?

Organic click-through rate is a good KPI to measure the quality of your website. If web searches like what they see, they'll dig deeper into your site. A CTR is a straightforward metric: It's the number of times a search result gets clicked, divided by how many views the search result has received.

Organic click-through rate is a good KPI to measure the quality of your website. If web searches like what they see, they'll dig deeper into your site. A CTR is a straightforward metric: It's the number of times a search result gets clicked, divided by how many views (or impressions) the search result has received.

To measure SEO success,
You can check for Organic traffic!
Compare the organic traffic with previous.
Check for the keyword position. The improved position will clear the scenario.

If your organic search are ranking in google 1st pages then you are successful in your work

Organic clickthrough rate is a good KPI for measuring website quality. If the web searches for what they see, they will dig deeper into your site. Clickthrough rate is a simple metric: it is the number of clicks on search results divided by the number of views (or impressions) received by the search results.

Correctly measuring the success of an SEO campaign can vary greatly depending on the type of business you're in and your objectives.
However, there are three key performance indicators (KPIs) that should always be considered when measuring an SEO campaign's effectiveness:

Rankings
Traffic
Conversions

Measure SEO Performance such as
Keyword Rankings. 
Backlink Quantity and Quality.
Organic Traffic
Time Spent on Page
Bounce Rate
Mobile Traffic
Click-Through Rate (CTR)
Domain Authority

There are many ways to measure your SEO success. We use to measure and analyze KPI's (Key Performance Indicators) to measure the success of a website. Few of those KPI are
Bounce Rate
Unique Website Visitors
Pages Viewed Per Session
Average Time on Page
Top Landing Pages
Top Exit Pages
Goals and Event Completions
Onsite Search Queries
Overall traffic

Getting lots of traffic is good but how much of that traffic can you convert is the real challenge. So, creating Goals to measure enquiries or leads generated is important to measure.