from someone who has some experience with immigration law:

I want to assure you that immigration judges are well aware of the potential for abusing asylum by making sham conversions. However, it is not the IJ's job to marshal evidence of a true religious conviction. As in the Article III courts, the IJs are there to provide a neutral venue for determining an alien's removability and any possible relief from removability under immigration laws.

The IJs have a bit more leeway than Article III judges when it comes to addressing respondents and witnesses, but it is ultimately the respondent's to demonstrate religious conviction--the burden is on the alien to establish eligibility for relief. Then, the DHS trial attorney [TA] may present evidence--including by cross-examining the respondent--demonstrating that the respondent does not sincerely hold the claimed religious beliefs.

Some standard questions asked by TAs to establish Christian bonafides:

Who is Jesus Christ?

What is your favorite story from the Bible?

What is your favorite prayer? Can you recite that prayer or a part of that prayer?

These seem like rather basic questions, but it is astounding how often Christian claimants cannot answer them. It is also fairly easy to spot the respondents who have been coached since they know one and only one story from the Bible, which is inevitably short-handed as: "water to wine." This is a reference to Jesus' first public miracle at a wedding in Cana and is considered an anti-Shibboleth by TAs and many IJs. Most frequently, Chinese applicants can name only this Bible story, as a result of being coached by the smugglers they use to enter the United States. Similarly, applicants who have been coached will say "the Lord's Prayer" is their favorite, and then recite some variation of the traditional Catholic grace before a meal (i.e. not the Lord's Prayer). The smugglers abroad and "immigration consultants" here in the United States are not very imaginative and have no real interest in their victims, so these responses do not change that often.

If the respondent satisfactorily answers these questions, the TA may then escalate to more complex questions of Christian theology. That is a matter of individual discretion, however. Often, you can get a pretty good idea as to the sincerity of the respondent's religious conviction with just those few questions. (Additionally, the TAs deal with many religions on a regular basis, not just Christianity. I suspect--without knowing, mind you--that they have a list of questions and answers prepared for the most common religions. They also prepare in advance of proceedings for this type of inquiry.)

You are right to point out that a person can know the theology of a religion, its practice, and its procedure without genuinely having religious conviction. As with many areas of law, we cannot actually know a person's state of mind, thoughts, and beliefs. However, a person's knowledge and behavior can be a good indication of same. Typically, an applicant for asylum making a religious claim will provide three types of evidence of belief: his testimony, the testimony of others who worship with him or see him at worship or know of his long-standing belief, and documentary evidence like baptismal certificates, photographs from religious ceremonies, etc. All three types are open to inspection and attack by the TA.