When Vice President Walter Mondale begins his maiden diplomatic voyage with visits to Common Market headquarters and the North Atlantic Treaty Organization here Monday, European leaders will be searching for clues about the news U.S. administration's plans to invigorate the Atlantic Alliance.

President Carter's intiative to dispatch Mondale on a tour of allied capitals has been widely applauded here. But traces of skepticism linger as to how the trip's display of solidarity will translate into policy.

NATO officials, in particular, have expressed concern about the Carter proposal to trim $5 billion to $7 billion from the U.S. budget. They seek reassurance from the vice president that such cuts will not undermine American military commitments in Europe.

Some allied countries, notably Britain, face severe financial pressures to lower defense spending. Only the United States and West Germany possess enough economic clout to pick up the slack and maintain NATO's military capability, assert NATO defense experts.

The same officials are worried that Carter administration's attempt to reduce defense spending coincides with a need for reaffirmed American ties to NATO. Their fears have grown with the alleged buildup of Warsaw Pact offensive might.

After meeting with NATO Secretary General Joseph Luns and the alliance's 15 ambassadors Monday morning, Mondale will travel across town for an afternoon session with the new president of the European Economic Community's executive commission, Roy Jenkis, and his 12 colleagues.

A fervent Atlanticist, Jenkins intends to emphasize his conviction that the Common Market's frequent trade disputes with the United States should not be allowed to interfere with close Atlantic ties.

Jenkins conferred with Mondale and Secretary of State Cyrus Vance during a private trip to the United States before Christmas. He assumed his four-year post here earlier this month.

With diplomatic niceties out of the way. Mondale and Jenkins are expected to plunge into discussion of plans for an economic summit conference among leading industrialized nations.

Most observes believe the summit will take place in London in late May or early June, timed with the NATO ministerial session there to enable President Carter to attend both meetings in one trip.

Jenkins also hopes to probe the new administration's views on reviving multilateral trade negotiations in Geneva. They have been stalled for several months over the issue of agricultural trade. The Europeans are seeking some protective measures for their troubled joint farm policy.

Another crucial issue Jenkins is anxious to broach with the vice president involves a more concerted Western approach in responding to demands of the developing world demands at a conference on International Economic Cooperation in Paris.

Backed by oil-producing countries, the developing nations have been pushing to link raw material prices to world inflation rates. West Germany and the United States have rejected condemn Western economies to a permanent inflationary sprial.

Other EEC countries fear that unless the West agrees on a more conciliatory stand, diplomatic pressures will build on the oil-producing countries to consider another round of oil price increases in the summer.