Is there any known issues for an existing Drupal 7 site migration into php 5.6?
For Drupal core specifically, that information will be in the issue queue. A quick search for "5.6" brings up no results. It it helps, I've been running D7 on 5.6 for ages now with no problems.
Contrib modules might be a different story, you'll need to check each of those individually, unless they offer that info specifically on the project page, or through a minimum PHP version specified in the module's .info file.
Any tools/modules to check the version compatibility of current Drupal 7 in PHP 5.6 ?
Not Drupal-specific tools (it wouldn't make sense for there to be since this just a plain PHP question). You can probably make good use of something like PHPLint. Personally I would start with IDE tools (switch the language version to 5.6, run a validator, then check the current issue list for errors)
Other than this any suggestion or advices prior to the migration ?
The only sane advice is for you to treat this as any other upgrade, whether that be code or server - make sure the site is fully tested in a development environment before pushing anything live. Have a plan in place to downgrade quickly on the live servers in case something goes wrong. If that's not an option (i.e. you don't control the server software), then re-double your testing efforts, and make sure server config is as identical as you can get it before pushing anything live.