There are a couple ways to retrieve a pre-shared key for a Cisco IPSEC VPN. The easiest way is to actually get it from the running config on the ASA. Unfortunately using a show run will only give you asterisks for the PSK, but you can use this command to see the PSK in the config:
more system:/running-config
If you have access to a working PCF file, you can also retrieve the key from that. The PCF file can be opened in your favorite editor to view and edit configurations, but the PSK is still not in a human readable form, but it can be decrypted using the tool at this site: http://coreygilmore.com/projects/decrypt-cisco-vpn-password/
The site above appears to be broken at the moment, but here is another site with the same capability: http://www.unix-ag.uni-kl.de/~massar/bin/cisco-decode