Now that Roy Williams has been picked to be president of the Teamsters, the biggest question is just how much power he will be able to exercise over the giant union.

The answer, according to industry and union officials familiar with the 66-year old Williams, will come at the Teamster convention in Las Vegas early next month. Specifically, the officials point to key elections and appointments in the union's Central States conference as a gauge of just how many "deals" Williams had to make within the union to win the presidency.

Although Williams only was elected interim president of the union earlier this month after the death of Frank Fitzsimmons, his election to a full five-year term at the convention is assured. Much less clear, however, is who will fill the union vice presidency and the powerful Central States post Williams vacated when he was elected president.

If Williams has his way, says a source familiar with the situation, he will pick someone from the Central Park conference to fill the vacant vice presidency, and his choice would be longtime aide Jack Yager.

In addition, Williams is believed to want Yager appointed to the key post of director of the Central Conference of Teamsters. These are key power posts within the 14-state conference and with Yager in place, Williams would be assured a continued power base. But there is speculation Williams may have had to promise the conference directorship to union Vice President Jackie Presser, leader of the Ohio Teamsters.

If someone outside the Central Conference is elected to the vacant vice presidency and someone other than Yager is appointed to the conference directorship, union and industry sources would see it as a sign Williams has had to give up substantial power to get to the top job.

Another sign to watch for at the convention, according to those familiar with the union, will be any attempt to ammend the Teamster constitution to re-create the post of general vice president. Ths is the post former Teamster President Jimmy Hoffa created in 1966 to position Fitzsimmons, then his top aide, to take over the union when he went to jail the following year. Fitzsimmons, who shunned the caretaker role Hoffa has envisioned, abolished the general vice presidency in 1971.

"If the general vice presidency is recreated, it will be created for Jackie Presser, and it will mean Bill Presser is still a formidable power in the union," said a source close to the Teamster situation. William Presser, Jackie's father, stepped down as a Teamster vice president several years ago to make room for his son, but still retains several union positions.

Another source long familiar with the union put it more bluntly: "If Jackie Presser is named general vice president, it will be very clear that the most serious kind of deal has been struck." He said, however, "it wouldn't surprise me if he had to strike a deal or two" to get the job.

Should Williams emerge from the convention with his power basically intact, sources predict he will move quickly to centralize his power within the union. Long a protege of Hoffa, Williams is expected to use his knowledge of the Teamster's national trucking agreement to cut across the union conference structure to solidify his power. After Fitzsimmons took over the union from Hoffa, he allowed much of the power to revert to the various Teamster conference leaders.

"You can safely predict you're going to have a very different kind of president than Frank Fitzsimmons," one source said. "Roy is very much of the same stripe as Hoffa." Several sources predicted that Williams would not make a formal move to centralize his power, but that it would occur on an ad hoc basis as he moved across conference boundaries to deal with contract problems. As with Hoffa, several sources predicted it would only be a matter of time before union and management officials were bypassing the conference structure and calling Williams to solve their contract problems.

Sources familiar with Williams over the years basically use the same words to describe him, words such as "tough-minded", "decisive" and "compulsive."