Netbook emergency router; default route question

[Ron Leach]

Using AspireOne netbook as a (temporary) replacement internet gateway 
router, combined with a proxy server.

Due to the layout of the building, using wlan0 as the connection to 
the internet (ADSL wireless router), and using wired eth0 port to 
serve the local network clients.  The intention is that clients will 
use the netbook as their gateway, through eth0, and the netbook will 
masquerade their traffic and then onward route to the internet over 
the wlan port.  Simple, basic, router, set up with 
system-config-firewall to masquerade eth0, the local net traffic 
[192.168.0.0/24].

The proxy server is needed so that some applications on other systems 
can temporarily route their traffic through the netbook, without 
having to reconfigure the default route settings on their hosts. 
Installed tinyproxy to do this.

But tinyproxy, though configured to send outbound traffic to the 
address of the ADSL router [192.168.13.1], appears to be placing its 
outbound traffic on eth0, not on wlan0.  Actually, tinyproxy doesn't 
'know' about ports, only addresses, so it is the netbook that is 
placing traffic for the ADSL router back onto the local net, instead 
of on wlan0.  I'm guessing, but I wonder if the default route is (for 
some reason) set to be reached using eth0 instead of wlan0.

Using network manager applet, I can see that both eth0 and wlan0 have 
default gateways and (of course) these are different because the two 
ports are on entirely different networks.

How do people check the default route?

Grateful for any advice,

regards, Ron