With the designation of an envoy to Peking, President Carter has filled all the open embassies of major import. So it is finally possible to take the full measure of his diplomatic appointments.

Before I assess their quality, however, it is necessary to sweep away a test that, though widely cited, is clearly bogus. I refer to the notion that the critical measure is the percentage of career Foreign Service officers named to embassies as against what are - with self-evident bias - deemed "political" appointments.

The distinction between "career" and "political" has been doubly falsified by the deep involvement of this country in foreign affairs since the war. One result is that many businessmen, lawyers, university and union officials and others have become highly conversant in international affairs. Another is that the diet of foreign business has attained a richness in political, legal, commercial, cultural and technical matters that go beyond the specialized training of the career diplomat.

In these conditions, the Foreign Service has become a kind of protective association assering the special privileges of ite members. To accept its definition of sound ambassadorial appointments is like accepting the view of the Teamsters on what constitutes good management of the trucking industry.

Rapid communication, moreover, has clearly reduced the importance of most embassies. So the true test lies in the caliber of the persons named to the very few ambassadorial posts that truly count in the formulation of national policy.

Paris, at present, is the most important of these. There is a strong prospect that the United Left, linking the Communists and Socialists, will win the legislative election set for next year. That would bring the Communists into government and create a constitutional crisis between the legislative majority and President Valery Giscard d'Estaing and the Gaullists under former Prime Minister Jacques Chirac.

An ambassador in touch with all three groups an capable of reporting objectively is essential. The Foreign Service officer named to the Paris embassy, Arthur Hartman, is perhaps the outstanding career diplomat of his generation. He has superb qualifications for the task. But he was chosen only after three others refused the job.

Moscow is also important, if only because the ambassador there has to supplement reporting on American viewpoints in a way that carries weight with the Soviet leadership. After running into trouble on the arms-control negotiations in March, the Carter administration, in the interest of maintaining continuity with the past, decided to keep the ambassador named by the Ford administration: Malcolm Toon, another career Foreign Service officer of high quality.

But previously Carter had sought without success to get former disarmament negotiator Gerard Smith to go to Moscow. The fact that the administration was plainly willing to remove Toon does not exactly heighten his clout with Soviet officials.

Peking important because by its choice of envoy Washington expresese the weight it assigns to the Chinese connection. Leonard Woodcock, the former auto-union leader who has been chosen to head the liaison mission in China, was an early Carter supporter who presumably commands attention at the White House.

Still, he seems to have been chosen chiefly because he breaks up the united anti-Communist front of American labor. Certainly he does not reassure the Chinese on the issue that really counts with them, which is American willigness to stand up to Russia. Somebody like former Undersecretary of Defense Paul Nitze would have probably said much more to Peking.

The more so as the President has sent to Japan a former senator prominently identified with a shrinking American role abroad: former Majority Leader Mike Mansfield. Mansfield wanted the China job, and was rightly turned down because he is so emphatically in favor of abandoning Taiwan to normalize relations with Peking.

But not being right for Peking does not make him ideal for Tokyo. On te contrary, though his political prominence signals the importance of Japan, his well-known views on the need to contract American power do little to serve this country's vital interest in pushing the Japanese into the mainstream of world affairs.

Finally, there is London, a critical post because Britain, which has loomed so large in American diplomacy, is now in obvious decline. The appointment of a new ambassador could have registered that decline. Instead Carter has chosen a prestigious figure from the academic world former Yale President Kingman Brewster.

What emerges from all this is typical Carter. No goofs but no inspired appointments either. An advance billing far in excess of actual performance. A number of good moves but no larger focus.