Usually, the most efficient way is to browse the Relative Index for that topic. Why? In the Relative Index the interdisciplinary number for a topic appears as an unsubdivided entry at the top of the list of entries for that topic (line 2 in the browse display, right after the immediately preceding line given for context in browse displays). For example, here is the start of the Relative Index browse display for "marriage":
By contrast, a simple search for "marriage" yields 74 records. The results are arranged in DDC number order, and the record for the interdisciplinary number (306.81 Marriage and marital status) is number 46 in the list; it will not appear on the first page unless you have raised the number of records to be displayed on the first page above the usual 30 records.
If you search instead for "marriage and interdisciplinary" you will get just the one record you want, with its class-here note "Class here . . . . interdisciplinary works on marriage . . . . ." That specific search is efficient enough to be useful; however, that kind of search works well only when "interdisciplinary works on [the sought topic]" appears in a class-here note or including note—and many records for interdisciplinary numbers have no such note.
For an example of another topic, here is the start of the Relative Index browse display for "pollution":
In this display, the entry for the interdisciplinary number (363.73 Pollution) is followed immediately by links to two Manual notes that can help you determine the best number for complex works about pollution.
By contrast, a simple search for "pollution" yields 135 records, and the record for the interdisciplinary number (363.73) is number 32 in the list. A simple search for "pollution and interdisciplinary" yields twelve records—not including the record for 363.73. If you display the full records, you will find that the second record (304.28 Environmental abuse) has the class-elsewhere note: "Class interdisciplinary works on pollution in 363.73." In this case, browsing the Relative Index is the more efficient way to find the interdisciplinary number.
For a third example, here is the Relative Index browse display for "garlic" (given in full because it is short):
A simple search for "garlic" yields six records:
With this search display in DDC number order, it is not obvious which number is the interdisciplinary number. To identify the interdisciplinary number record, you could display the full records to find the record with the Relative Index entry "Garlic" unsubdivided; that would be record number 5 in the list, the record for the built number 641.3526Alliaceae—edible bulbs. A search for "garlic and interdisciplinary" yields no record. In this case also, browsing the Relative Index is the more efficient way to identify the interdisciplinary number.
Here are examples of works for which the above-mentioned interdisciplinary numbers can be used:
A History of Marriage: From Same Sex Unions to Private Vows and Common Law, the Surprising Diversity of a Tradition (classed in 306.8109 History of marriage and marital status, built with 306.81 plus T1—09 History)
Pollution: Opposing Viewpoints (classed in 363.73 Pollution)
Growing and Using Garlic (classed in 641.3526Alliaceae—edible bulbs)