Buffer Size Checking

I'm planning on using highland streams to perform some network operations in node.js, but I expect some of these operations to be slow in nature. I also have some real-time sources that spit out data in bytes. What's the best way to check the current buffer size? I'd like to change my application behavior based on the back-pressure. If the buffer gets too great, I'd like to manually control how these sources are outputting data so that varying degrees of network connectivity don't affect my application.

Thanks