I was looking for an answer to this question today and stumbled across a page on the MaximumASP site, which indicates that looking at the first few characters of the CSR will provide the answer.
I still want to do a bit of research to verify that what they have said is true, but it seems to be, based on my limited test. I generated CSRs for 512-, 1024-, 2048-, 4096-, 8192-, and 16384-bits* and examined the first few characters. My generated CSRs for 512- and 1024-bits matched the MaximumASP page. Here are my results.