Unfortunately by default the Netgear DG834 / DG834G v1,v2,v3 doesnt allow you to check from the web interface if you have interleaving applied on your line. However, thanks to Roseway who tells us how to find out this information by telnetting into the router.

First logon to the router and then select this address:
http://192.168.0.1/setup.cgi?todo=debug

which should result in a screen which just says 'Debug enable'.
(That's assuming that you haven't changed the router's IP address from the default).

Then exit from the web interface and open a command-line window. Type: telnet 192.168.0.1

You should get a BusyBox welcome message to confirm that your telnet connection is established.

Now type: cat /proc/avalanche/avsar_modem_stats

This will give you a stream of information about the ADSL connection. Scroll up and find a section something like this:

[Upstream (TX) Interleave path]
CRC: 0 FEC: 0 NCD: 1
LCD: 0 HEC: 0

[Downstream (RX) Interleave path]
CRC: 0 FEC: 0 NCD: 0
LCD: 0 HEC: 0

[Upstream (TX) Fast path]
CRC: 1 FEC: 68 NCD: 0
LCD: 0 HEC: 0

[Downstream (RX) Fast path]
CRC: 49 FEC: 0 NCD: 0
LCD: 0 HEC: 0

In this example the interleave figures are all zero, apart from the first NCD value, which always seems to be 1, so this router is in fast mode. If you see zeros in the fast path sections and other figures in the interleave sections, then interleave is on.

DG834G v4

v4 unlike its predecessors has a broadcom chipset and therefore needs a different command.
After selecting debug mode http://192.168.0.1/setup.cgi?todo=debug

telnet into the shell telnet 192.168.0.1 as above but use the following command

# adslctl info --stats

which should give you the following info:

Status: Showtime Channel: FAST, (or interleaved).