It’s true that Search Engine is a primary traffic source to websites. For making SEO Friendly, web pages have to be optimized as per search engine’s webmaster guidelines. Webmasters can use free tools avalable online test the website for further optimizing the site for not only search engine but also visitors.
Google trends helps to perform keywords research using this trending searches, comparing trends by search terms, forecast, regional trends, etc.
Link Google Trends
A free AdWords tool, Keyword Planner aids to get keyword ideas, search volume and traffic forecasts for selecting bettwe and relevant keywords.
Robots.txt testing tool in Search Console for testing the robots.txt file.
Link Robots.txt Testing
W3C validator check the markup validity of Web documents in HTML, XHTML, etc.
Link HTML/XHTML Validator
Use the Structured Data Testing Tool to check that Google can Correctly parse your structured data markup and display it in search results.
Google Mobile-Friendly Testing Tool check the website’s mobile friendly status.
Link Mobile Friendy Tool
Use Page Speed Insight to find out how to make your web pages fast on all devices.
Link Page Speed Tool
Google Search Console is a free service offered by Google that helps your monitor and maintain your site’s presence in Google Search results.
Link Google Console
Check Cascading Style Sheet (CSS) and (X)HTML documents with style sheet.
Link CSS Validator
taken from pinterest.com