In addition to RusselB's event-based script, you can always scan mIRCs "internal address list" for wildmask matches (Below's a short example script).

There are several methods to evade "matches" anyway: if you check for *!user@host for example, a clone with a different username won't match; "dynamic IPs" = host addresses changing frequently; BNCs and other proxys; vHosts...

Use "/ialcheck" or "/ialcheck <fullmask (wildmask)>".
Code:
alias ialcheck {
  var %mask = $iif($1,$v1,$$input(Enter a wildmask (nick!user@host) to check in $&
    your internal address list for,eg,Find matching uses in internal address list))
  var %t = $ial(%mask,0), %n = 1
  echo -agc info There are %t matches for mask $qt(%mask) $+ .
  while ($ial(%mask,%n).nick) { 
    echo -ag  >> %n : $+($chr(2),$v1,$chr(15)) ... $ial(%mask,%n)
    inc %n
  }
}