Our pitch email subject line was a brief description of the product then "- Free demo codes for your readers". The body had a similar format: what we do, here's your code, contact us with any questions, thanks.
Lots of sites have feedburner badges to give subscriber counts. We found one blog that was clearly faking that though. Their traffic to us was way lower than an 80k subscriber blog would send.
The number of comments on posts is a good indicator too. Anything greater than 0 means they've got a fairly big audience.
Nice hack and something I appreciate because it's such a simple thing to do.
"... Lots of sites have feedburner badges to give subscriber counts. ..."
Did you try to obtain the stats from each site?