Searching for deleted/skipped numbers

Hi, I'm not sure if this is possible .... but .... I've created a database with tables that are joined by a unique id number. Sometimes we delete rows and would like to find unused id numbers to use again. Is there away to query an integer field for skips in a range? e.g. if I search a field of integers ranging from 1 to 100 can I find the unused numbers? Make sense?, think I'm repeating myself. Any directions or suggestions would be great! Thanks in advance, Joe Arcieri