One of the best ways to identify the longtail keywords is finding out your current website traffic details and associated keywords
You can use the tools like Google Search Console, Google Analytics
It doesn’t matter what kind of website you have, LTP will help you to find the best keywords that you can use for your blog to drive more traffic and generate more revenue in whatever methods you choose.”
Harsh Agrawal, ShoutMeLoud.com
Most people use the google keyword planner to find the long tail keywords but it is limited for the paid search
How do the Long Tail Keyword Tools find long tails keywords?
What are the sources they look to find those keywords?
Before using these keyword tools you have to find out the answer for that?
I’m a fan of Long Tail Pro! It’s powerful keyword research tool that I’ve used and am very impressed. I definitely recommend Long Tail Pro. It’s a must-have tool for any SEO or Internet Marketer”
Pat Flynn -smartpassiveincome
You can use the Ubersuggest keyword finder tool free of charge but you can buy the paid version if you want advanced features.