5 Ways You Can Be Swindled by Click Fraud Pay-per-click advertising is a good way to make an honest living but a very tempting target for crooks all over the world.

Ever since pay-per-click advertising was introduced in the late '90s, there have been unsavory characters lurking on the Internet that devalue the online ad model via click fraud. In its early days, click fraud was often someone clicking an ad a couple extra times.

These days, however, click fraud techniques have become so complex that the activity can often slip under the radar of the most vigilant advertisers. Protect yourself—and your revenue—against click fraud by familiarizing yourself with the strategies and the red flags that indicate their presence in your PPC campaign.

Related: 6 Tips for Preventing Click Fraud

1. A series of computers are infected by malware, resulting in a botnet. This technique is perhaps the most malicious type of click fraud, employing a network of computers infected with malware and following instructions from a centralized controller.

These botnets can comprise countless computers, like the well-known ZeroAccess that has been active since 2009. As of August 2013, ZeroAccess infected almost 2 million computers. Some researchers estimate that this botnet costs advertisers over $2.7 million per month.

These are among the hardest to detect, as clicks come from various IP addresses and can be very human in their activity. A sign of a bot is clicks from a user who has JavaScript disabled, although online tools from click fraud experts are the best bet in tracking down illegitimate clicks. Fruition's new Click Fraud Tool, which is available for early access via sign up before its Black Friday launch date, is one such tool.

2. A website is utilizing hit inflation attacks. Hit inflation is another malicious form of click fraud that attacks a website by redirecting legitimate users to an advertisement, resulting in a "click." These websites bounce users to the advertisement and then back to the page they were actually trying to view. Even though the user may not have even seen the ad, the page visit counts as a click.

Hit inflation attacks are hard to detect because the traffic does come from legitimate users. A specially-made online tool to detect click fraud is the best bet to detect it.

3. Someone is employing people on a click farm. In this method, someone pays an individual (or large group of people) money to click on ads. These click farms are most often found in developing countries, where wages are low and there are many willing people. Click farms teach employees how to emulate the behavior of real Internet users so that the clicks are inconspicuous.

Many clicks originating from countries outside the business' operations are a good sign of a click farm, as well as traffic hidden behind proxies or multiple clicks from the same user.

Related: Stop Click Fraud

4. The publisher (or someone else) is persuading users to click on an ad in exchange for a reward. This is called "incentivized traffic,'' where a publisher offers up some sort of reward—like a discount code or points for credit on a website elsewhere—just so that a visitor to the sites clicks on an ad. Some users of this technique simply pay the users a small amount to click on the ad, similar to a click farm. These are low-value clicks as the user is not interested in the ad, just the reward.

5. Someone encourages people to click on an ad with messages. Just like incentivized traffic, this encourages Internet visitors to click on an ad although there is no reward offered for the click.

Messages bordering the ads encourage users to "click to support the website." Although it seems innocent enough (if you like the website, you should support it, right?) this practice is forbidden by Google and is treated as fraud. In fact, the well-known website SheKnows.com committed click fraud via crowdsourcing in 2012 when its editors encouraged its staff to click on ads.

Before engaging in your next PPC campaign, it's important to be aware of click fraud and the forms it takes. Other signs that click fraud is prevalent:

• Hundreds of clicks from one user in an impossible amount of time, like one second
• A large number of clicks from a single user in either a short or long amount of time
• A large number of clicks from users hiding behind proxies

Even though there are plenty of avenues to damage the pay-per-click advertising, the PPC model is still a viable way to generate awareness, leads and revenue, provided you are on the lookout for thieves.

Related: Put the 'Pay' in Pay-Per-Click