MT-LXX parallel

[SMacG]

For a selected word in the LXX, I know that the parallels function gives a number of MT equivalents.   For a given MT equivalent, I know how to find verses where the two match.   But the parallels function also provides a number of instances in which there is 'no lexical form' equivalent - i.e. there may not be a Hebrew word for the selected LXX word.   How do I identify these situations (and only these situations), particularly when there may be multiple numbers of MT equivalents to the selected LXX word, and many verses to search through?