Insurance companies investigate all injury claims to varying degrees. Pre existing conditions are nearly always discovered if not in exhaustive discovery of medical records, then through 'independent medical examinations'. Insurance companies at the beginning of a claim will ask for signed authorizations allowing them to get the claimant's medical records. If the claim can not be settled, then most courts will in fact allow insurance company attorneys to obtain signed authorizations entitling them to do reasonable investigations into a claimant's prior medical and physical history.
Indexing: Prior Insurance Claims
The insurance companies also have a central indexing bureau that notifies them at the commencement of a claim as to any and all prior claims by that claimant. The indexing system keeps track of and reports all kinds of claims, even if only for a property damage claim or a single medical bill payment after an accident.
The fact that a person may have had previous issues does not prevent a claim. In any case, the negligent party is liable for all damages that represent an aggravation of any pre existing condition.
One investigative tool is that, yes, insurance companies often do video surveillance, the results of which can severely harm a claim. Video cameras are now very small and of much better quality than they have ever been in the past. Any claim can be investigated, but video survellance is in fact often used on workers compensation claims. For example, if the claimant has made a statement to a claims adjuster or anyone else handling the matter that they can no longer reach groceries on the top shelf or carry grocery bags to and from the car, they should not be surprised later, to see a video of them in the grocery store, reaching a bottle of spaghetti sauce off the top shelf, and/or loading bags in and out of the car. I've seen that, and many other, such videos.