In California, the first Horticultural Commissioners were established in 1881.  I do not know when border inspections started but the passage below was penned by John Steinbeck.
 
Bob Roach
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Much earlier I spoke of the changes at state lines, changes in Highway
English, in prose forms on the signs, changes in permitted speed.  The states'
rights guaranteed under the Constitution seem to be passionately and gleefully
executed.  California searches vehicles for vegetables and fruits which might
carry pernicious insects and diseases, and regulations of these are
enforced with almost religious intensity.  
 
Some years ago I knew a gay and inventive family from Idaho.  Planning to
visit relatives in California, they took a truckload of potatoes to sell along
the way to help pay expenses.  They had disposed of over half their cargo when
they were stopped at the California line and their potatoes refused entrance.
They were not financially able to abandon their potatoes, so they cheerfully
set up camp on the state line, where they ate potatoes, sold potatoes,
bartered potatoes.  At the end of two weeks the truck was empty.  Then they
went through the inspector's station in good standing and continued on their
way.
 
John Steinbeck
Travels With Charley, 1962
 
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>My wife's book group is reading "Lolita" and they came across a passage >about the pair driving through Arizona and California, where at the border "a >policeman's cousin would peer with such intensity at us....[and ask] Any >honey?"
 >   Do any of you know whether there were such inspections 40 years ago >and if so, what would they be looking for?   Or is this just part of Nabokov's >imagination?