At the moment QuickBooks Online Payroll it is not possible to see an employees accrued holiday. We're always looking at ways that we can improve our payroll features and we can understand this could be beneficial for our customers. Please leave this as feedback on this link which can then be explored by our product engineers who can then look at the idea of implementing it into a future update to our current payroll product.
You can add a separate rate of pay as holiday on a payslip for your employees. To do this go to the employee section and find the employee you wish to add holiday pay for, then in the bottom right hand corner you will be able to see a green + icon. There you will have options for Added pay, Deductions and Leave. Select Added pay and then when you choose an option you will have the ability to rename this as 'Holiday pay' or however you wish to record this. Then when running your payroll it will give you the option to add the amount on this pay rate which will be under a separate heading on your payslip.
Please let us know if there is anything else we can help you with.