Lets say that Google, Yahoo and MSN the only search engines, what You need to keep in mind, optimizing Your site SEO. Yahoo is definately one to be represented in.
1. Most accurate way is to query directly in Yahoo:
2. Some websites, reporting YBL
3. Setup account in Yahoo Site Explorer, but You have to bear in mind that its takes ages, before Yahoo Site Explorer really activates ans starts statistics.
There are slight differences in backlink number reporting, so its wise to use only one meter to analyze data over long time.