Develop expectations for current year trade payables [by supplier / type of purchase / purchasing division / cost centre etc].
Compare actuals to expectations.
Investigate variances over xxxx (% of performance materiality, $ amount) from expectations and obtain explanations and corroborative evidence for these variances.
Where there is no appropriate explanation or corroborative evidence supporting the variance, consider the need to plan and undertake additional work and/or document any errors identified.