The syslog messages showing the building of UDP/TCP connections will show both the original source IP address and the PAT'd address, so this will tell you the info you want.
From the Logging messages reference (http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_63/63syslog/pixemsgs.htm):
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302013
Error Message %PIX-6-302013: Built {inbound|outbound} TCP connection number for interface_name:real_address/real_port (mapped_address/mapped_port) to interface_name:real_address/real_port (mapped_address/mapped_port) [(user)]
Explanation A TCP connection slot between two hosts was created.
Where:
connection number is a unique identifier.
interface, real_address, real_port identify the actual sockets.
mapped_address, mapped_port identify the mapped sockets.
user is the AAA name of the user.
If inbound is specified, then the original control connection was initiated from the outside. For example, for FTP, all data transfer channels are inbound if the original control channel is inbound. If outbound is specified, then the original control connection was initiated from the inside
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