As others have mentioned, on Linux, you can look in /proc but there are, depending on your kernel version, one or two limits:
First of all, the environ file contains the environment as it looked when the process was spawned. That means that any changes the process might have made to its environment will not be visible in /proc:
$ cat /proc/$$/environ | wc -c
320
$ bash
$ cat /proc/$$/environ | wc -c
1270
$
The first shell is a login shell and initially has a very limited environment but grows it by sourcing e.g. .bashrc but /proc does not reflect this. The second shell inherits the larger environment from the start, which it why it shows in /proc.
Also, on older kernels, the contents of the environ file is limited to a page size (4K):
$ cat /proc/$$/environ | wc -c
4096
$ env | wc -c
10343
$
Somewhere between 2.6.9 (RHEL4) and 2.6.18 (RHEL5) this limit was removed...