list anaconda packages for which I specified version number
There is no any programmatic way to extract the information you are looking for. The only way I could think of is by extracting the history of the commands you typed while installing your packages through your terminal (in case of mac or linux machines) or command prompt (in case of windows machine).

For checking the history of typed commands in MAC or LINUX:
Type history in your terminal window
For checking the history of typed commands in Windows:
Type doskey /history or press F7 key inside your command
prompt

After extracting this history of commands you typed, you can create a piece of code to check for commands in which you have specified version numbers. 
How can I unspecify a package version if I do not want to specifically use that version but I would rather like to get the latest possible version? You can just do pip install [package_name] --upgrade in case of pip or conda update [package_name] in case of conda to upgrade a package to its latest version
Is there a way to list the packages for which I specified the version number. For example, if I specified scipy version but not numpy version by installing them like this: conda install scipy=0.15.0 numpy, I would like to be able to know that I specified version 0.15.0 for scipy. Furthermore, how can I unspecify a package version if I do not want to specifically use that version but I would rather like to get the latest possible version?