You can use $nick(#,$nick).pnick to return $nick's mode prefixes in $chan, e.g. ~@+nick if the user was +q, +o and +v.
Since +q/~ is the highest mode it should always be first if the user has it, so if ( $left($nick(#,$nick).pnick,1) == ~ ) { dostuff } should work.
You should bear in mind that there is probably a server somewhere that allows ~ in nicknames (here, perhaps). $left($nick(#,$nick).pnick,$+(-,$len($nick))) would return just the modes, without the nickname.