How do you know an allegory is valid?

There are two parts to the question:

   Is allegory appropriate for a particular verse.
   Is the proposed allegory valid.

On the first question there are these views:

   Enumeration - Allegory is only appropriate where a NT author mentions it as an allegory.
   Sample - NT authors mention allegory as a sample of what we may find. If we find it, it is appropriate.
   Sensus plenior - every verse has a hidden meaning, so it is always appropriate.

On the second question there are these views:

   Sample - An apparent allegory is valid if it teaches truth. Many allegorical interpretations can derived from a single portion of scripture and they each may be valid as long as they teach truth in agreement with the Bible. This is the source of differing opinions and much debate, since there is no agreement on what Biblical truth is. Allegory is generated to fit personal interpretations of Biblical doctrine.

   Sensus plenior - An allegorical meaning must be the same everywhere the term exists in all scriptures. As such, the scriptures are viewed as a giant crossword puzzle. Opinion has no place, only tentative solutions until the whole is resolved. Opinion, by definition, is a position held when there are insufficient facts to determine truth. In sensus plenior, an allegory is resolved by adding more facts to the discussion. A statement made under the 'banner' of sensus plenior is subject to all the rules associated with sensus plenior.

Application:

   Sample - Opinion is pitted against opinion until someone declares a rule of faith and anathematizes the other.

   Sensus plenior - Drash is practiced where all portions linked by various Remez methods are overlaid to make a single picture. The picture is then interpreted Christologically.

Boil

There are 24 verse which use the word bashal and are translated as boil, seethe, ripe, sodden, roated, baked. All of these verses must make theological sense when the sensus plenior is unpacked from them using the same allegorical meaning for bashal.

When two or three examples are given, and no contradictory examples are available, a solution is penciled in, subject to scrutiny under all the other verses that are linked by Remez.

It is impractical to justify every word in every conversation concerning sensus plenior since the linked scriptures expand exponentially as the sensus plenior for each occurrence are examined.

As such, a formal discussion of sensus plenior would include in a challenged allegorical meaning, the sensus plenior exposed in a Remez-linked passage where the proposed allegory did not work.

Without getting into the details in this article: The narrative of Eli's son's and their three pronged meat fork, contains an account similar to Adam's rib, in much more graphic detail than the Genesis account of the birth of the church through Christ's death. This would be offered in an extended discussion of how boil represents tribulation. As each of the other verses containing the word are unpacked, an incontrovertible solution is derived due to the interlocking of all the allegorical meanings that must be consistent.

Types of errors (may not be exhaustive):

Observation - missing details.

Remez - bad links because of using English rather than Hebrew

Drash - missing portions

Interpretation - preconception biases

These can be discussed dispassionately and objectively.