I focus on frames that are roughly my size while I'm browsing eBay and have had enough of my own, so that I can rather quickly tell the proportions between the main tubes to figure things out approximately. What throws me for a loop are larger or smaller frames with awkward proportions that sometimes make them seem to mirror what I usually see in my size range, which is between 60-62cm. It most frequently happens with slightly smaller frames that have high bottom brackets and longer-than-usual head tubes to compensate for the rise.

It's just a matter of looking at bicycles way too much, really... That's all!

-Gregory