What is the legal ruling for resorting to the bihs'a?

There is no basis for trial by bish'a[1]in Islamic law. Rather, we must follow the words of the Messenger of Allah who said,"The claimant must offer evidence [for his claims] and the defendant must swear [to his innocence]" [recorded by Ad-Darqutni]. This noble hadith sets the manner of seeking and establishing truth and denying false claims.

Muslims must adhere to this injunction and reject any offensive practices which have no basis in Islamic law. The only manner prescribed by Islamic law for verifying claims of usurped rights are confessions, proof and the like.
Allah Almighty knows best.

[1]Trial by bish'a involves the suspect licking a red hot iron. A scarred or burnt tongue or the degree of burns determines the suspect's guilt.