ABC News' Paula Faris reports:

The Chulews - Mike, Rebecca and 2-year-old Veronica - recently went on a summer vacation for practically free.

Their gas, flight, hotel and even their meals were about 80 percent off. That's because the Chulews of Texas are mystery shoppers.

"If I wasn't a mystery shopper," Rebecca Chulew said, "I don't even know how I would pay full price for these things."

Click here to learn about accredited mystery shopping companies. Watch: 'Travel hacking' secrets to traveling super cheap. Related: Secret shopping sites look to put money in your pocket.

The Chulews said being a mystery shopper was like going on a secret spy mission.

Mystery shopping companies hire people to grade everything, from free newspapers for guests and towels laid out for the beach to whether the hair dryer in the bathroom works.

Shoppers then document their findings and opinions on a score sheet.

"We're always looking for mystery shoppers," said David Lipton, president of the mystery shopping company Sensors Quality Management. "The most successful mystery shoppers are those who are reliable, responsible. They have a keen eye for detail. They can follow the instructions that we provide."

When the Chulews recently stayed at a hotel in Corpus Christi, Texas, they noted that the pillows in the lobby were messy and that there was sand on the carpeted steps.

At the end of their stay, though, the Chulews gave the hotel a thumbs-up. The hotel declined to comment to ABC News.

The Chulews said they'd performed about 150 mystery shops this year, saving them about $5,000 in expenses they would have bought anyway.

"My haircut was a mystery shop," Mike Chulew said. "My clothes. My shoes. My socks."

"Once you know you can get a half-priced pair of shoes for doing 20 minutes of paperwork, why not?" Rebecca Chulew said.

"[Mystery shoppers] will save a significant amount off of their airfare, their stay at a hotel, a night out at a restaurant," Lipton said.

But there are scams out there, he added. Be wary of companies that ask you to pay them money upfront.

Make sure you are dealing with a reputable company. Here's some advice from the FTC on how to avoid a scam. And there's a mystery shopper trade group that provides web resources.

Mystery shopping is just one way, though, to save big on a vacation as the summer comes to an end.

• Christine Partello of Boston and her fiance saved $75 on their honeymoon this summer by using Yapta. The website tracks the price of your flight and sends a notification if it drops.
• Gail Presses of New Orleans saved $600 by booking her hotel through Tingo. It tracks prices and automatically rebooks your room when the price drops.
• Rahul Razdan of New Jersey used Zalyn to find a car rental coupon, saving his family $246.