So how Execryptor detects OllyDbg?
isdebuggerpresent,
ZwQueryInformationprocess
FindWindows
OpenProcess and scanning export table of parent process
thread local storage
and lots of other tricks
you can read about some of them in andrea geddons blog in reteam
as well as in some threads at woodmanns
also there is a sample crackme in crackmes.de
and kao has written a solution on it
hope it would be enough info for starters