How can I test for plugins and only include them if they exist in .vimrc?

You can wrap that block in a conditional which uses the exists() function to check if a variable, command or function defined by the plugin is known to vim.

Here are a couple bits that I have in files under ~/.vim:

" after/plugin/speeddating.vim
if exists(':SpeedDatingFormat')
    SpeedDatingFormat %-d %B %Y
endif

" ftplugin/ruby.vim
if exists('g:loaded_surround') && !exists('b:surround_'.char2nr(':'))
  let b:surround_{char2nr(':')} = ":\r"
endif

Note that the above bits are in files that get evaluated after normal plugins, here an ftplugin, and a file in the after/plugin directory.

Another option would be to use try/catch blocks, although this requires at least vim 7.0:

if v:version >= 700
    try
        runtime bundle/pathogen/autoload/pathogen.vim
        call pathogen#infect()
    catch
    endtry
endif

Once something in the try section of that block fails it will skip to the catch section. Since the catch section is empty, it will just continue on with the remainder of the initialization file after the endtry statement.

Since this is manually loading code rather than relying on a plugin being already loaded, this can be done in the .vimrc file itself.

My preferred method is just to check for the existence of the plugin file. I find this simpler.

if !empty(glob("path/to/plugin.vim"))
   echo "File exists."
endif

I wanted to achieve this while keeping my Vim configuration together within .vimrc, rather than in a bunch of after/ directories. This is the solution I came up with:

1. Check each plugin's existence by checking for any single command that it provides with exists(), and set its options if it does exist. (This is just like in the accepted answer.)

2. Put all the options set in the above manner within a function (called SetPluginOptionsNow() in my code).

3. Call this function on the VimEnter event, which is triggered while entering a new Vim session - but crucially, after the plugins have all been loaded. Because of this fact, our exists() checks can check for the plugin functions without a problem.

Here's a sample from that part of my .vimrc.

""" PLUGIN-SPECIFIC OPTIONS
" These are "supposed to be" set in after/plugin directory, but then
" cross-platform synchronization would get even messier. So, au VimEnter it is. 

function! SetPluginOptionsNow()


    " NERDTree Options
    if exists(":NERDTree")
        map <F10> :NERDTreeToggle<CR>
    endif

    " Syntastic Options
    if exists(":SyntasticCheck")
        let g:syntastic_stl_format = '[Syntax: line:%F (%e+%w)]'
        set statusline+=%#warningmsg#
        set statusline+=%{SyntasticStatuslineFlag()}
        set statusline+=%*
        " and so on...

endfunction

au VimEnter * call SetPluginOptionsNow()
""" END OF PLUGIN-SPECIFIC OPTIONS