You can use lsf.str.
For instance:
lsf.str("package:dplyr")
To list all objects in the package use ls
ls("package:dplyr")
Note that the package must be attached.
To see the list of currently loaded packages use
search()
Alternatively calling the help would also do, even if the package is not attached:
help(package = dplyr)
Finally, you can use RStudio which provides an autocomplete function. So, for instance, typing dplyr:: in the console or while editing a file will result in a popup list of all dplyr functions/objects.