If you omit the top cell name on the stream in form, it will import ALL the cells in the stream file. So you really don't need to know the cell name up front. If you specify the top cell name, it only imports that cell and all the cells within its hierarchy, so can be used to import a subset of the file.
You could use the "ascii dump" option of import stream to look at the contents - and from this infer whether any particular cell has been referenced or not - if there are no references, it's likely to be the top cell.
But it may just be simpler to import the whole file, and then look to see what is a likely name for the top cell. Or just ask whoever provided the GDS file!