mPath <- strsplit(Sys.getenv("PATH"), ";")[[1]]
mPath[grep("Rtools", mPath)]
# [1] "C:\Temp\Rtools\bin"              "C:\Temp\Rtools\gcc-4.6.3\bin"
# [3] "C:\Temp\Rtools\gcc-4.6.3\bin32" "C:\Temp\Rtools\mingw_32\bin"

library(Rcpp)
evalCpp("1+1")

library(installr)
install.Rtools()