Suppose you've determined what testing you need done by a qualified testing firm. Now, how do you select the firm? Because you’ll trust this firm to do dangerous work quickly, accurately, thoroughly, and safely, use a much higher standard of care than what applies to selecting the lawn care firm for your facility. Don't put out three bids and select the lowest one. Instead, approach this as if you were hiring an employee. Use basically the same methods to ensure you hire a highly qualified firm.
Examine the relevant training, experience, and certifications. Discuss with them their work methods so you can determine if you are hiring a safety-conscious, procedure-based team that pays attention to details. Contact references to confirm what the testing firm tells you.
Don't make the mistake of writing up a huge, complex contract or project schedule and then trying to find someone to work to it. The working documents will be most beneficial to you when they result from consultation and collaboration with the testing firm. Talk about the specific work you need performed, and ask how they recommend doing it.