requirements allows you to impute required packages from R code, files (like .R, .Rmd and .Rnw) and directories. It primarily looks for use of functions like library() and ::, but also uses heuristics to detect common implicit dependencies (e.g. methods, shiny, and roxygen2).
The inspiration for this code comes from packrat. We're extracting it out in to a separate package so that it can be more easily extended and used by others.
requirements is not currently available on CRAN, but you can install the development version from GitHub with:
# install.packages("devtools")
devtools::install_github("hadley/requirements")