My question is the following, if I contract a hosting that supports laravel, this should already have the composer already installed, or should it be installed?
How can I realize that my hosting service really supports laravel 5.5 from linux console by ssh connection?
$ which composer.phar
$ which composer
/usr/local/bin/composer # if installed you should see something like this
composer isn't really part of any core Linux distro, so you may have to install it. If you really want a one-and-done setup, that is what Forge is for: