From the seedy tradition of Sam Spade comes a spiffy new kind of private eye: the business investigator.

This new breed has eased onto the corporate scene in recent years, offering to handle the often-gritty undercover investigations that executives are too uncomfortable, unskilled or busy to do themselves. Probe a business deal that looks too good to be true, for example. Or examine a potential partner's background.

These investigators report business is flourishing, for a variety of reasons.

First, a string of recent frauds and hoaxes has loosed a flurry of corporate housecleaning. Not only are businesses hiring private detectives to doublecheck the resumes of potential employes, but some long-time employes also are being scrutinized, particularly if they are in line for a promotion.

"For heaven's sake, no one wants the newly named president or vice president to turn out to be a high-school dropout with a prison record," groans one personnel manager.

Second, rising costs are forcing business to crack down on petty theft in the office as well as the factory floor. Often, the booty is not simply a couple of pens and note-pads, but spare computer parts or hospital laboratory supplies. Construction companies, in particular, are complaining more about the disappearance of small tools and lumber from worksites, say these investigators.

One Florida company hired a business investigator to look into an $8.5 million cost overrun on one project, for instance.

The investigator found that construction managers were padding equipment rental budgets in return for kickbacks from rental companies. In fact, the managers had rented so many backhoes that "they ran out of dirt to move and had to move the same piles of dirt back and forth."

Brokerage firms are relying in creasingly on outside investigators to help scutinize promoters of new stock issues.

The top-flight business detective is a far cry from the gumshoes of movie and novel. Many firms have lawyers and accountants on their staffs and would turn up their noses at traditional snooping jobs such as wife trailing.

And they are not cheap. Fees are computed either on an hourly or job basis. Some typical ones for the lmost common jobs handled by business investigators (expenses are extra): routine employe investigation (for a secretary, or foreman, or low-level manager), $75-$100; investigation for a top-level executive, $250-$300; routine corporate investigation: $50 an hour; investigation of a stock promoter: $200-$400.