Judging by evidence, and by an oath along with one witness

It is narrated from Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Producing proof and testimony is the duty of the claimant, while taking the oath is binding on the one against whom the claim is made, that is, the defendant."
Imam Ahmad bin Mani' (may Allah have mercy on him) narrated this hadith with his chain through Muhammad bin al-Hasan bin Abi Yazid, Hajjaj and Amr bin Shu'ayb, from his father, from his grandfather.

It is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) issued the judgement that "when the claimant has no proof (evidence or witness), the defendant (the one against whom the claim is made) is bound to take an oath." These are the words narrated by Imam Ishaq.
The wording of the other narrator (Muhammad bin Bishr) is: "Whoever makes a demand without witnesses, the one from whom it is demanded has the greater right to take the oath."
Imam Ishaq also stated that Abu Mu'awiyah narrated to us from Hajjaj with this same chain a hadith like it.

It is narrated from Muhammad bin Sirin (may Allah have mercy on him) that Shurayh (may Allah have mercy on him) used to say: "Two upright witnesses should testify that you two (i.e. the seller and the buyer) parted by mutual consent after the sale or after the option of the session (the choice to keep or cancel the sale); and if there are no such witnesses, then it is his (the claimant's) oath by Allah that you two did not part by mutual consent after the sale or after the option of the session."
Its chain is sahih (authentic), and this saying is mawquf (stopping) at Shurayh (may Allah have mercy on him).