Is there any online tool or trick to find the Sitecore version from customer facing site?
As per my knowledge, No we can't find the Sitecore version is used for building the website from customer-facing site. The "No" here for the finding website using the browser or by visiting the website. But yes we can find the Sitecore version if we have delivery website folder by looking into the Sitecore.kernel dll version, see here for version - Can you identify the Sitecore version from the Sitecore.Kernel.dll version?
Of course, if you have access to the file system, you can look at
/sitecore/shell/sitecore.version.xml, or you can log into desktop mode and from the start menu, go to All Applications > System > License Details if you have access of it. There are few other ways but none of them are for the devilry website - https://kb.sitecore.net/articles/891209