the only way you could is to /whois the nick, maybe set a var for that nick once you get the line that the nick is registered. that's usually raw 307. you could do like:
Code:
raw 307:*:{ set $+(%,identified,$2) true }


then you'd end up with a var like:
%identifiedVenoman true

then could do in your check:
Code:
if ($($+(%,identified,$nick),2) == true) {
  do something
}


if you're dealing with a lot of nicks, you'd need to /whois everyone on join (would that flood you off? maybe. so timers). In which case you'd get flooded with /whois responses, which means you need to change the raws for these (and maybe more, depending on the network you're on):
311,307,615,616,319,312,317,318

hope this helps.

-Venoman