halacha - Is checking for bugs halachically required?

Various methods of checking for bugs in produce, i.e., lettuce, are officially recommended by kashrus agencies and other halachic professionals. However, is use of one/any of these methods--or even a formal check--halachically required? Or is the halachic requirement simply that one not eat bugs?

MOTIVE: For certain mitzvot, such as taharas hamishpacha and Pesach preparation, checks are halachically required, and--at least for taharas hamishpacha--one must perform them in a certain way. (Moreover, for taharas hamishpacha, the check is sufficient to establish the condition halachically; here, I ask if the same is true with regards to checking for bugs.) I am wondering if the same is true for food. Is a check [of a certain kind] halachically necessary?

IMPLICATIONS:

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  Could a self-designed method of checking be halachically acceptable? What level of reliability would it need to have in order to be so?

  
  


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  From whom can one learn a method for checking for bugs? Could one adopt the method suggested by, say, someone with a lower kashrus standard, but who still keeps kosher?

  
  


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  Could no method (that is, no checking) be acceptable if one knew the lettuce to be free of bugs?