In order to use the exit status of mutiple commands as the condition of an if block, use begin; ...; end and the short circut commands and and or.

The exit status of the last foreground command to exit can always be accessed using the $status variable.

if test -f foo.txt
        echo foo.txt exists
else
        echo foo.txt does not exist
end

will print foo.txt exists if the file foo.txt exists and is a regular file, otherwise it will print foo.txt does not exist.