What is included?
Ask what the quote covers: hours, attempts, reports, travel, and expenses. Ask what would make the price go up.
How do they charge?
Investigators often bill hourly plus expenses. Process servers often charge per service, with extra for rush or repeat attempts. Security agencies usually bill per officer, per hour, with a minimum shift.
Deposits and cancellations
Ask whether a deposit is needed, whether it is refundable, and how much notice a cancellation needs. Get all of it in writing.
Compare like with like.
Ask every professional the same questions so their quotes are easy to compare.
Read the hiring checklist