Are there tools available for Government personnel to validate the billable expenses executed by contractors while TDY (Labor hours, Travel hours, lodging, plane ticket, misc expenses).
Great question. Absolutely there is and in fact most contracts require this.
I'll assume this is not a contract for construction.
Almost every payment clause has a paragraph or subdivision that mentions "other supporting documentation required..." to make payment. So take a look at what payment clause you have in your contract (look for FAR 52.232-XX clauses or DFARS 252.232-XXXX clauses, or FAR 52.216-7 Allowable Costs and Payment if cost reimbursement or Time-Material CLINs). Additionally look for FAR 52.232-25 Prompt Payment in your contract, it should be there unless your contract is a commercial contract (format). FAR 52.232-25(a)(3)(x) "any other information or documentation required by the contract (e.g., evidence of shipment)." If your contract is silent on this, then it may require a modification to include it. Talk with your contracting officer, I'm quite sure they will work with you on this.