Until 2003 looking at license plates one could trace not only a diplomat, but a foreign company representative, too. Today "foreigner" plates are issued only to diplomats.

These plates are white on red, of a "three digits-one letter-three digits" format. The first three letters stand for the country where the car owner is from (see the list below). The letter (either "D" or "T") stand for a diplomat or trade representative respectively. The last three digits are free numerals. There is a regional section and the "RUS" inscription at the right part of the plate. No flag posted onto the plates.

Heads of diplomatic representations have different plates of a "three digits-two letters-one digit" format, where the first three digits stand for his/her country, two letters (always CD) stand for "Chef de diplomatie" and the last digit is a free numeral (I have never seen it exceeding 2).
Useful Links:
1). Images
2). Regional Codes
001 United Kingdom
002 Germany
003 Canada
004 USA
005 Japan
006 Spain
007 France
008 Belgium
009 Greece
010 Denmark
011 Italy
012 Luxemburg
013 Netherlands
014 Norway
015 Turkey
016 Australia
017 Austria
018 Algeria
019 Egypt
020 Rwanda
021 Argentina
022 Afghanistan
023 Myanmar
024 Bolivia
025 Brazil
026 Burundi
027 Ghana
028
Bangladesh
029 Guinea
030 Zambia
031 Peru
032 India
033 Indonesia
034 Jordan
035 Iraq
036 Iran
037 Ireland
038 Iceland
039 Cambodia
040 Kenya
041 Cyprus
042 Congo
043 Costa-Rica
044 Kuwait
045 Laos
046 Liberia
047 Lebanon
048 Libya
049 Mali
050 Morocco
051 Mexico
052 Nepal
053 Nigeria
054 Venezuela
055 New Zealand
056 Pakistan
057 Burkina Faso
058 Senegal
060 Somalia
061 Sudan
062 Sierra Leone
063 Thailand
064 Tanzania
065 Tunisia
066 Uganda
067 Uruguay
068 Philippines
069 Finland
070 Sri Lanka
071 Chad
072 Switzerland
073 Sweden
074 Ecuador
075 Ethiopia
076 Angola
077 Zaire
078 Columbia
079 Cameroon
080 Guinea Bissau
081 Portugal
082 Bulgaria
083 Hungary
084 Vietnam
086 Poland
087 North Korea
088 Cuba
089 Mongolia
090 China
091 Romania
093 Yugoslavia
094 Benin
095
Gabon
096 Guiana
097 Mauritania
098 Madagascar
099 Malaysia
100 Niger
101 Singapore
102 Togo
103 CAR
104 Jamaica
105 Yemen
107 Palestine
108 Nicaragua
109 Mozambique
110 Eq. Guinea
112 Malta
113 Cape Verde
115 Zimbabwe
116 UAE
117 Cote D’Ivoire
118 Namibia
120
Oman
121 Qatar
124 South Korea
125 Chile
126 Panama
127 Israel
128 Macedonia
129 Albania
131 Vatican
132 Lithuania
133 Syria
134 Estonia
135 Latvia
136 Bahrain
137 South Africa
138 Armenia
139 Georgia
140 Saudi Arabia
141 Slovenia
142 Uzbekistan
143 Kyrgizstan
144 Croatia
145 Azerbaijan
146 Ukraine
147 Moldova
148 Czech Republic
149 Slovakia
150 Belarus
151 Tajikistan
152 Turkmenistan
153 Kazakhstan
154 Guatemala
155 Bosnia and Herzegovine
156 Eritrea
157 Paraguay
158 Georgia
159 Brunei
499 European Community Comission
500 EBRD*
501 United Nations Information Center
502 EU Commission
503 League of Arab States
504
IBRD**
505 International Monetary Fund
506 International Migration Organization
507 Red Cross and Red Crescent
508
Intl. Committee of the Red Cross
509 International Finance Corporation
510 UNIDO
511 UN High Commissar for Refugees 
512 UNESCO
513 African National Congress
514 IBED***
515 International Investment Bank
516 Intersputnik
517 ICSTI^
518 ISTC^ ^
520 International Labor Organization
521 Interelectro
522 CCE^ ^ ^
523 Interstate EC Economic Committee
524 European Space Agency at RF
525 Eurasian Patent Organization
526 Taibei-Moscow Commission
527 CIS Headquarters
*     European Bank for 
       Reconstruction and
       Development
**    International Bank for
       Reconstruction and
       Development
***  International Bank for
       Economic Development
^     International Center for
       Scientific and Technology
       Information
^^  International Science and
      Technology Center
^^^Commission for
      Computing Equipment
Notes: