How to tell which colorscheme a Vim session currently uses

You can set the Vim color scheme by issuing

:colorscheme SCHEME_NAME

but, oddly enough, you can't get the currently used scheme by issuing

:colorscheme

as this results in "E471: Argument required". I also don't see the color scheme listed in the output of :set.

So how do you go about figuring out the current color scheme in use (other than manually switching the themes until you recognize it)?

There's no guaranteed way (as a colour scheme is essentially a load of vim commands that are sourced). However, by convention there should be a variable g:colors_name that is set to the name of the colour scheme.

Therefore, try this:

echo g:colors_name

If you get E121, it's either a poorly made colour scheme or it's the default one.

A shinier way of doing this is (for recent versions of vim):

function! ShowColourSchemeName()
    if exists('g:colors_name')
        echo g:colors_name
    else
        echo "default"
    endif
endfunction

Then do:

:call ShowColourSchemeName()

If it says "default", do :colorscheme default and see if the colours change. If they do, you're using a malformed colour scheme and there's not a lot you can do about it other than manually switching themes until you recognise it.

The variable g:colors_name is documented here:

:help colorscheme