Вы можете использовать режим «сухого хода», который просто претендует на загрузку и установку пакетов
$ aptitude install -sy xlockmore
The following NEW packages will be installed:
xlockmore
0 packages upgraded, 1 newly installed, 0 to remove and 27 not upgraded.
Need to get 194 kB of archives. After unpacking 373 kB will be used.
Would download/install/remove packages.