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()
try
echo g:colors_name
catch /^Vim:E121/
echo "default
endtry
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