Eugène François Vidocq, a French solider, started the first acknowledged private investigating business in the early 1830s. Historians credit Vidocq with enterprising many techniques still in use today, such as shoe prints and ballistic evidence. Many copied Vidocq's idea, but early private investigators acted more like mercenaries, often intimidating workers attempting to unionize, and private police, when the sanctioned police were ill-equipped to handle a case. The P.I. remained largely inaccessible to the average public until the boom era of the 1920s made Americans wealthy enough to hire their own P.I., turning the industry into what we see today.
Before a P.I. considers some of the more expensive forensic methods available, he may search for physical evidence around the suspect's residence. In cases of infidelity, the P.I. may ask the spouse to search for certain signs. The P.I. may search for receipts thrown in the trash, indicating gifts were purchased for a secret lover or instruct the client on where to find such evidence.
Cellphones and computers have complicated our lives, but new technology may make the job of a P.I. much easier if the suspect does not implement standard privacy measures. For example, a P.I. may set up an online account for a person's cell phone, using his information, if the owner fails to do so, allowing access to texts and phone records. Another simple intelligence technique simply requires the P.I. to install a "keylogger" on a computer, which records passwords and gains access to email accounts.
The advancement of global positioning systems, GPS, are another area of technology that makes covert intelligence cheaper and easier for modern day investigators. A cheap cellphone with GPS capabilities planted in someone's car can track a person's movement from the comfort of a computer using a GPS locator subscription service.
Sometimes a P.I. can not attach a GPS tracker to a car, or the other techniques fail. In this case they use the old-fashion technique of spying on an individual. Typical surveillance uses a pair of binoculars, an unmarked and inconspicuous car, facial disguises for physical pursuit and an audio/visual device to record someone in the act of deviancy.