If your first inclination is to think the best of people, you may want to avert your eyes. Today's topic is glass-half-empty stuff. We're looking past Dr. Jekyll to Mr. Hyde.

That's because of a presentation I attended recently in Pleasanton that focused on the many ways unscrupulous folks take advantage of the rest of us, ranging from identity theft to telephone hoaxes to bogus auto repairs. A "Senior Scam Stopper" seminar sponsored by state Sen. Ellen Corbett targeted older folks -- about 75 turned out -- but many of the warnings apply to all ages.

Roland Esquivias of the California Public Utilities Commission addressed anyone with a telephone when he touched on two common scams: slamming (a change in providers without customer consent) and cramming (unauthorized charges). He advised inspecting monthly statements and demanding answers for anything that looks strange.

He described the "star 72" scam, in which a victim is told a loved one has been hospitalized and can be reached by dialing a number that begins with *72. That activates call forwarding and enables crooks to use the account to place unlimited long-distance calls. Free vacation packages are offered in the "809" scam, in which victims are told to dial a number with that area code and are charged for every minute they're on the line.

Joaquin Murphy of the Bureau of Automotive Repair said to be suspicious of extra services recommended by auto repair shops. His investigative unit recently enlisted 150 owners to take their cars in for oil changes and see what else technicians suggested.

"Most of the time they were recommending anywhere from $200 to $1,200 worth of additional services that weren't necessary," he said.

If you check your owners guide for manufacturers' recommendations, you'll find motor oil in today's cars generally needs to be changed after 7,500 miles, not 3,000. Power steering fluid hardly ever needs to be flushed, and transmission fluid rarely does. In fact, changing it sometimes is bad.

"If you put a generic transmission fluid in a car that requires a specific type, it can do damage," he said.

Jane Kreidler of the Contractors State License Board spoke to all homeowners when she warned against hiring unlicensed contractors. They are not bonded, so homeowners are liable for property damages. They carry no workers' compensation insurance, so customers can be sued for injuries incurred.

Always get three bids, she said, never pay cash, and state law prohibits down payments of more than of $1,000. Be sure your contract stipulates a work time line, payment schedule, trash disposal and purchase agreement for materials. ("If the contractor doesn't pay the supplier, the supplier can put a lien on your house," she said.)

Pleasanton Police Det. Michael Rosillon warned of crooks intercepting mail or rifling through trash to steal personal and financial information. Alameda Deputy District Attorney Cheryl Poncini talked about crooks masquerading as animal control officers in search of poisonous snakes to distract residents while accomplices burglarized their houses.

"If crooks would use that energy for good, we would have a better world," she said.

I'm not saying the presentation eroded my trust in mankind, but I did check my pants pocket to see if my wallet was still there.

Contact Tom Barnidge at tbarnidge@bayareanewsgroup.com.