From what I've read main difference between V1 and V2 is memory capacity.
Maybe they list them in the specs? Thats the only way I can think of.
Per specs listed on NE it says "Featuring 480 MHz MIPS 74K CPU, 8 MB Flash and 64 MB RAM," those specs are the v1
V2 has 128MB RAM and 128MB NAND flash.
Just want to add I have the 3500v2 basically same as the 3500l but without usb port. Bought it 2 years ago as a refurb works great and I luv the parental controls only issue is the wireless connect using that green button and getting a code (forgot what they call it) never worked.
There are also physical differences on the back see link below
http://openlinksys.info/images/3500lv2/IMAG0363.jpg