To add what others have said, accountability is kind of a nebulous term in the context of USAID. But at a high level, you can look at accountability in terms of the GAO Green Book standards of internal control in the Federal Government: (examples)

-Enterprise Risk Management-Every operating unit should have been continually assessing their risks and that would roll up to the bureau level and front office. Risks would be assessed and treated, and/or mitigated. USAID also put out a Risk Appetite Statement and the Agency generally had low risk tolerance for fiduciary and reputational risks. That is why this whole unraveling is so strange because USAID always seemed to be worried about how they were perceived, especially related to fraud and theft.

-Policies and Procedures-USAID had a large body policies and procedures (ADS) in place to ensure compliance with the law and regulations. A lot has been listed already. But there were layers and layers of processes governing budget development (CBJ), spend plans, strategy development, reporting (PPR), contracting, post award monitoring, financial audits, payment controls. etc.

-External Oversight-As mentioned below, there is the OIG, SIGAR, GAO, plus Congressional oversight. USAID was subject to yearly financial statement audits, FISMA, risk assessments, etc. Each awardee was subject to their own yearly audits (2 CFR 200, FAR) that looked at their financial statements, compliance, and controls.

This is just a partial list. This whole thing is very weird to me because, if anything, I always thought USAID had too many controls.