Customs officials have developed a system for targeting containers that might hold illegal drug shipments.

They analyze data and cargo patterns and check past histories. They inspect cargo and container documentation, and they use common sense.They say Miami leads the nation in drug seizures. Out of 43 cocaine and marijuana busts since October 1986, 20 occurred at the Port of Miami, say Customs officials in Washington. Seven other seizures were at other Florida ports.

It's impossible for Customs to screen every container for possible drugs, even every container that comes from South America, say officials. It can take anywhere from two hours to a full day to inspect a single container. In total, about 50,000 containers come into Miami annually.

In one case, a container was so tightly packed with furniture, it took virtually all day for a team of inspectors to unload it. But once they did, they found a false wall and 6,900 pounds of cocaine.

Customs has 500 inspectors specially trained to screen contraband at all major U.S. ports. An average port of entry might examine four to six containers a day.

Customs, therefore, must choose its spots carefully. Essentially, it is a process of elimination, they say.

For instance, there was the case - perhaps apocryphal - of a container cargo of toilet paper from Spain coming into Miami from Honduras. The circumstances of that shipment led Customs agents to believe that a thorough inspection might be in order. It's said to have resulted in a cocaine bust.

Many times, the physical condition of the container itself is a giveaway: tampered or non-existent seals, an absence of markings, illegible markings, film over a number or other indications of alterations.

A proper Brussels International Container Code will identify a probable legitimate container. The BIC code is a recognized international method of numbering containers.

The Customs agency rarely receives a hot tip from an anonymous source that leads to a container seizure, officials say.

One question a Customs agent must ask himself is whether the method of transportation and the cost warrants the particular handling of goods. Worthless junk packed in I-beam-reinforced cases might merit further investigation.

Then there is all the documentation that comes with any shipment. Between the dispatch of the consignment and acceptance or filing of entry documentation, as many as eight documents may be involved, including the bill of lading, consignment notes, container packing lists, cargo manifests and service contracts.

At the time of clearance, several other pieces of paper come into play. Among them are goods declarations, inspection certificates, import/export licenses, visas and payment documentation.

This is where the human factor and the computer factor merge. Customs uses its Automated Commercial System to hel preview the documentation for anomalies in information, handling requests, indications of unusual insurance coverage, cost/value discrepancies and type-overs and erasures.

In addition, the agency uses the information to study industry trends for patterns that aren't in the norm.

Customs also coordinates other intelligence sources within the agency and other federal agencies and has sought better information sources from overseas and commercial entities.

Customs has recently entered into carrier agreements with major shipping lines that enable it to get more information. The agreements also affirm that a carrier is taking positive steps to prevent drug smuggling opportunities.

If a company has one of these agreements on file, its containers might not require the intense scrutiny given those of a company without an agreement.

Anything that helps in reducing the number of possible container targets is welcomed. As P.T. Wright, an inspector and national program manager in Customs' Office of Inspection and Control puts it, Customs is faced not just with the issue of finding a needle in a haystack, it's which haystack.

But no matter how many documents or agreements are filed, finding the drugs always comes down the physical examination of a container.

The question of fining a shipping company is a delicate issue. Officials are reluctant to discuss the situation in detail, but they note it boils down to a question of care.

If the proper care is not manifested by the shipping agent, he can be held liable, said one. Customs looks at the amount of effort a line has taken to thwart narcotics smuggling attempts: what procedures have been put in place, whether documentation is consistently sloppy, whether containers are always overloaded or overweight.

Coast Guard Commandant Paul A. Yost said that even with all the effort his agency puts into drug interdiction - at a cost of approximately $400 million a year that the Coast Guard spends - only 5 percent to 10 percent of the drugs coming into the country are seized. Others estimate that about 15 percent of drugs reaching the country are seized.

Customs says it is probably doing about as well, percentage-wise, in

finding drugs in containers.

Mr. Wright of Customs' Office of Inspection and Control says the basic message Customs wants to send to the abusers of containers and the carriers is that these means of transport provide a very great opportunity for a drug smuggler, and Customs is aggressively pursuing the problem because of the tremendous volume of drugs that can be put into any container.