How could a person find out if they have been named a beneficiary in a trust but that information is being kept from them?

Truthfully and unfortunately, this is very difficult. Trusts by their very nature are private. You could bring a lawsuit against the trust, and it would have to be provided as part of discovery. You, also, could threaten to bring a lawsuit, saying that if you did they would have to provide the trust document and that everyone could save a lot of money, time and stress, if they simply provided it without you taking that step.