Leonard Fuld is a spy. He doesn't wear long trench coats and dark glasses. But he does spend time leafing through the Yellow Pages of phone books, peering at satellite pictures of company plant sites, and, on occasion, studying cardboard boxes.
Yes, cardboard boxes. They can give important clues about the volume of shipments a company may make of a certain product.
Mr. Fuld is president of Information Data Search Inc. of Cambridge, Mass., a detective agency of the business world. He is one small part of a growing global effort by businesses to keep tabs on competitors.
This is not the nefarious and illegal world of industrial espionage and theft of technological secrets for foreign companies or governments - something the Reagan administration, among others, is urgently trying to stop.
Instead, it is a legal game of ''I Spy'' - the often mundane, but increasingly important, world of business intelligence gathering. A click of a camera at a trade show or a news clip from a technical journal can greatly affect a company's strategy.
The Japanese are the acknowledged masters of intelligence gathering. They have been known to go so far as to measure the rust on a railroad track outside a company's plant to determine the volume of shipping. Japanese companies are helped - particularly in technical areas - by the Ministry of International Trade and Industry, which observers say acts as a central clearinghouse for information.
Company executives concede that there's often a thin line between ethical and unethical practices. In a few cases, companies have illegally acquired information - such as when Hitachi officials admitted stealing computer secrets and hardware from IBM.
But many businessmen say there's little need to engage in questionable activities, since plenty of information is readily available through sources such as the public library.
''You don't have to fly upside down in a biplane to get this kind of information,'' says Fuld. ''It is a common-sense business.''
Businesses, to be sure, have been ''spying'' on one another since the beginning of commerce. In Colonial America, for instance, textilemakers would hire youngsters to walk behind competitors' clothing carts to scout out the latest designs.
Today, however, the corporate ''I Spy'' game has become more scientific, more intense. Mounting competition in the world marketplace is forcing American firms to sharpen data-collection practices. Information itself has become an important commodity.
Some observers contend that the 1950s were the years of sales, the '60s and ' 70s the years of marketing, and the '80s and '90s will be an era of intelligence gathering and competitor analysis.
''Many companies have come to realize that competitor analysis isn't something you do as an occasional part of planning,'' says Michael Porter, a professor at the Harvard Business School and an authority on the subject. ''It's part of running the business.''
Already many companies, at the tap of a computer keyboard, can call up more than 300 worldwide ''data banks.'' Within a few hours, these can help paint a portrait of a company right down to subtle changes in pricing policies. Firms increasingly rely on this information for strategic planning.
Other cubbyholes of information are readily available. Consider, for instance , the sleuthing sources used by Information Data Search. Its traditional outlets include public libraries, annual reports, technical magazines, credit services, and data bases.
But there are other more creative sources of information. Among them:
* Corrugated boxes. If you can find out who supplies a company's shipping crates, it may help to pinpoint the sales volume and manufacturing activity of a firm - provided that the crate producer is willing to give out information on the number of boxes it sells to a client company.
* Yellow Pages and help-wanted ads. These can reveal how a company markets itself as well as the types of people it's hiring. It can also give an indication of the growth rate.
* Aerial photographs. Enlarged pictures of a plant over a period of time can give clues about a company's expansion activity.
* The plant watch. A favorite tactic of firms for years has been to hire business school students in the summer to sit outside a competitor's plant, binoculars in hand, and count the number of workers going in and out the building. It can, in some cases, give accurate estimates of the size of a work force.
Still, data collection is only one part of competitor analysis. The difficulty for many companies is choosing what's important and how to use it. It is in this area - intelligence analysis - that observers believe American firms hold an edge on the Japanese.
''The Japanese gather a lot of data, but I question how good their analysis of it is,'' says James Roden, director of business research for the General Telephone & Electronics Corporation.