I am a new Debian / Ubuntu Linux sysadmin. I would like to find out what version I would install if I ran the apt-get command. How do I find out which versions of a package can I install using apt-cache/apt-get command? How do you check package version using apt-get/aptitude on Debian or Ubuntu Linux server?

There are various commands or options that can tell you about the version number on Debian or Ubuntu Linux based system.

Method #1: Check package version using apt-cache command

The syntax is:
apt-cache policy {package}
OR
apt-cache madison {package}
For example, before I install nginx package I would like to know what version of nginx I would get on my system, run:
$ apt-cache policy nginx
OR
$ apt-cache madison nginx

Method #2: Check package version using apt command

The syntax is
apt list {package-name-here}
apt list vim
Sample outputs:

Listing... Done
vim/xenial,now 2:7.4.963-1ubuntu5 amd64 [installed]

I have vim package # 7.4.963 version installed on my system.

Method #3: Check package version using aptitude command

The syntax is:
aptitude versions {package-name-here}
aptitude versions {package-name-here} | more
aptitude versions {package-name-here} | awk { print $2 }
To find htop version you are about to install on Ubuntu server, enter:
$ aptitude versions htop
Sample outputs:

Package htop:                        
p   1.0.3-1build1                                                                               xenial                                                                   500 

Package htop:i386:
p   1.0.3-1build1                                                                               xenial                                                                   500 

Method #4: Package simulation using aptitude or apt-get command

The syntax is as follows:
aptitude -V -s install {package-name-here}
apt-get -V -s install {package-name-here}
No action taken. It perform a simulation of events that would occur based on the current system state but do not actually change the system. Of course, press n when prompted to avoid installation.

A note about finding version of installed packages

Use the following syntax:
apt-show-versions {package-name-here}
OR
dpkg -s {package-name-here}
dpkg -s {package-name-here} | grep -i version
To see version of install package called vim, run:
$ apt-show-versions vim
vim:amd64/xenial 2:7.4.963-1ubuntu5 uptodate
vim:i386 not installed
OR
$ dpkg -s vim | grep -i version
Version: 2:7.4.963-1ubuntu5
Vim is an almost compatible version of the UNIX editor Vi.
This package contains a version of vim compiled with a rather
version of Vim. See the other vim-* packages if you need more