The Supreme Court ruled today that police officers need a warrant to track suspects with a GPS device, but experts say civilians still have a rich resource of spy tools at their fingertips.

The justices voted unanimously against digital tracking in one of the first major cases to test constitutional privacy rights in recent times. Lawyers for the District of Columbia lost their argument that attaching a barely noticeable GPS tracking device to a car's undercarriage was too trivial a violation of property rights to have any bearing under the law. Despite the prevalence of CCTV and GPS technology in smartphones, the court ruled that members of the public have every right to expect their trips to the grocery store remain secret.

Though it's illegal for civilians to monitor others by tagging them with GPS devices without their knowledge, there are ways for people to track their spouses, friends and children that don't involve spyware. Chester Wisniewski, senior security advisor at Sophos, an online security consultancy, says it's easier than ever for civilians to track friends and family: "If you had a small cellphone and accidentally dropped in your wife's purse, you could track her anywhere." Secretly tagging an adult with a GPS tracking device could result in legal hot water, says Mari J. Frank, an identity theft expert and privacy lawyer based in Laguna Niguel, Ca. For example, if your spouse is having an affair, he may be able to sue for invasion of privacy, Franks says. "If it's an ex-spouse, you could file a criminal case for stalking."

Here are several legitimate ways to keep track of others.

1. Sign your kid up to an app

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Keeping tabs on one's children -- especially when they're behind the wheel of a car -- is a challenge for many parents. Wayne Irving, a father of four children and president and CEO of Laguna Niguel, Ca.-based technology company Iconosys, thinks he has found a way to give kids freedom -- with invisible strings. He invented an app called My Max Speed. The trade off: Parents hand over the keys of the car and their child downloads the app on their phone (there are several other similar apps out there including Footprints for iPhone and FaimlyMap from AT&T). My Max Speed tracks the child's exact speed and location through GPS data every five seconds. One downside, say experts, is that this app and others are based on trust; if your child shuts it off, you lose track.

2. Befriend your friends online

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Experts say one of can be the simplest ways to monitor someone's whereabouts is using Facebook, Twitter, Foursquare and Google Latitude, which all give accurate locations for those who sign up. For example, Google Maps on Facebook shows who's been checked in (virtually, with pictures) and allows users to look at a photograph of the place. A recent poll by MetLife Auto & Homes showed that 15% of Americans use social media to say that they left their home, while 35% of Americans between 18 and 24 years of age "check in" our Tweet about their location. They may not always tell the truth, experts say, as it's easy to check into a fancy restaurant from the sidewalk. Others warn that the resource is ripe for burglars: 74% of ex-burglars say they believe location services like Google Street View is helpful to burglars planning raids, according to an infographic by CreditSesame.com.

3. Tag your child (literally)

Kelli B. Grant

A growing number of devices let consumers keep tabs on keys, pets and -- although they weren't designed for such use -- even their kids. Keeping track of an adult using such messages may be illegal, but Wisniewski says the rules are different when it comes to a parent or guardian keeping track of a minor under 18 "to ensure their health and safety." A host of protection services and apps promise to find lost smartphones, tablets and other gadgets, using either GPS or logging a location when the device connects to the Internet. There are also GPS-enabled tags for pets, and a new radio-tag system that can be attached to pretty much anything -- including people. But experts say neither technology is perfect. If Fido slips out of his collar, the Tagg system can't help, and the Bikn radio tags only work within 800 square feet.

4. Sign up to a GPS-tracking phone plan

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Parents have a number of features available to them through software and carrier controls to monitor teens' cellphone use and location, as well as their online activities: this can keep track of a child who likes to spend money playing games online or protect him/her from visiting inappropriate websites. T-Mobile, for example, offers a "FamilyWhere" to let subscribers find a family member's location, and $20-a-year Zone Alarm Social Guard monitors Facebook posts. Although many of the programs are designed to be unobtrusive, meaning the person being tracked might never notice, experts advise telling teens that they're being monitored, and why, to foster good behavior and trust.

5. Hire a private detective

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It's the most obvious -- if expensive -- way to track a person. However, many private eyes aren't as handsome, nor do they have the same witty one-liners, as fictional private detectives in TV shows. Plus, it may take longer than one hour to solve a puzzle. Still, they do a brisk trade for insurance companies trying to detect fraudulent claims. (Hint: Don't go bungee jumping with a claim pending for a neck injury or take a trip to the beach when off work on sick leave.) According to International Counterintelligence Services or ICS, rates vary depending on experience of the private investigators and the jurisdiction of the case. Fifty to 60 hours of work by a P.I. would cost around $6,000, according to Ben Walker, case analyst with ICS. Also, there are organizations like the California Association of Licensed Investigators that have a code of ethics relating to filing accurate reports, not over-charging clients and staying within the law. But Frank says this is a grey area. "If the private investigator makes you fearful and harasses you," Frank says, "that's a different story."