Unless the company that uses/used the building has dumped toxic material on the roof the main concern would be asbestos. If there is an asphalt based roof on the building the asbestos fibers could be either in the field of the roof or in the flashings around the penetrations or at the perimeter. Finding that out requires taking samples to a lab. If the tests come back positive a licensed abatement contractor needs to get involved. Some roofers are also licensed abatement contractors. If asbestos is found before the roofing bids are done then the additional removal costs can be identified up front and will likely be less costly than if found during roofing.