How do the lexicographers determine at what point a participle may also receive a nomination for becoming a preposition?

Looks like they survey millions of pages of current examples.

"Taken as a whole, these Reading Programmes represent one of the most extensive surveys of the English language ever undertaken. Since the first publication of the Oxford English Dictionary, the breadth of materials available and the means of retrieving and analyzing those materials have expanded incalculably. Despite the changes, the original aim of the programme remains unaltered since the days of James A. H. Murray: to collect examples of the changing vocabulary of English from a highly diverse range of published sources spanning the entire English-speaking world, and to provide the Oxford English Dictionary's editors with a constantly updated and ever more detailed record of English past and present."

http://dictionary.oed.com/about/reading.html