How can I check for available entropy?
This is not a solution, but a clarification of how entropy is gathered and used in Linux.

Linux actually has two different entropy pools:

/dev/random and /dev/urandom.

The former is a true random pool, fed by system entropy sources.
The latter is more of a PRNG such as those found on the BSD's and OS X.

However, even urandom requires a seed of 'real' random entropy in order to produce quality pseudo-random data. On recent kernels, a complete lack of entropy in /dev/random will still not block urandom, but urandom will re-use the last valid seed until more entropy becomes available. Because urandom is nonblocking, most services that require a steady stream of entropy use it rather than relying on /dev/random.

There are still some services however, such as various SSL suites, which can't make do with pseudo-random entropy, but require a truly unpredictable entropy source. In this case, urandom (or any other PRNG) can't be used, and /dev/random comes into play.