Verifying that the attendance you were invoiced for happened
Time-and-materials invoices are usually paid on trust, because verifying them after the fact is nearly impossible.
An invoice arrives for six hours of an engineer's time on a Tuesday. The work was done in a plant room, unaccompanied, three weeks ago. Nobody in the organisation can now say whether it was six hours or three. For a product-side comparison related to verifying attendance records, see employee attendance tracking software before deciding which data the operation actually needs.
This is not primarily a question of dishonesty. Estimates are made from memory on both sides, travel is treated inconsistently, and rounding conventions differ. Without a record made at the time, the invoice is the only evidence and it is one-sided.
Sign-in that is actually used
The oldest control is still the most practical: contractors sign in and out at a defined point, recording arrival, departure, the asset attended and the work order number. For a wider operational and compliance reference, consult U.S. Department of Labor hours-worked guidance.
It fails when signing in is inconvenient, unenforced, or done retrospectively at the end of the visit for both times at once. It works when there is a single entry point, someone whose job includes checking it, and a stated rule that unrecorded attendance is not paid.
Attendance recorded without a job reference cannot be matched to an invoice line. Time and date alone leave you knowing someone was there without knowing what for.
Electronic options, in proportion
Where volumes justify it, contractor sign-in via a tablet at reception, a QR code at the plant room, or an app with a location check at the site boundary all produce a timestamped record without a person having to police a book.
The proportionate version verifies presence at the site at the moment of arrival and departure. Continuous tracking of contractor staff is neither necessary nor, in many jurisdictions, straightforward — they are somebody else's employees and the data protection position is the contractor's responsibility as much as yours.
Require the detail on the invoice
Specify the required breakdown in the contract: date, work order reference, engineer, arrival and departure times, hours claimed by category, travel stated separately, materials itemised.
An invoice that says 'maintenance works, £1,840' cannot be checked by anyone, and accepting one establishes that unverifiable invoices are acceptable. Reject the first one on format and the problem generally does not recur.
Agree the conventions in advance
Most invoice disputes are about conventions rather than facts. Is travel charged, and from where? Is there a minimum charge, and does a twenty-minute job consume it? How is time rounded, and in whose favour? What rate applies to a job that starts at four and finishes at six?
Written down at contract stage, these are administrative details. Discovered on the third invoice, they are a negotiation conducted from a weak position, because the work has already been done.
Sample rather than audit everything
Checking every invoice line in detail costs more than it recovers for most organisations. Checking none establishes that nobody is looking.
A monthly sample — a handful of lines traced back to sign-in records and job records — is enough. The purpose is not to recover the value of the sample but to maintain the accuracy of everything else, and a contractor who knows attendance is periodically reconciled prices and invoices differently from one who knows it is not.