Test this code here.
Code:
alias isadmin {
  if (!$isid) return
  set -l %elevated $false
  set -l %a $ticks $+ $rand(1111,9999)
  if ($com(shell $+ %a)) .comclose shell $+ %a
  .comopen shell $+ %a WScript.Shell
  if ($comerr) goto end
  if (!$com(shell $+ %a,Run,1,bstr,cmd.exe /c whoami /groups > %a $+ .tmp,int,0,bool,true)) goto end
  if ($read(%a $+ .tmp,w,*S-1-16-12288*)) %elevated = $true
  :end
  if ($com(shell $+ %a)) .comclose shell $+ %a
  if ($isfile(%a $+ .tmp)) .remove %a $+ .tmp
  return %elevated
}