Omega_Void
Your Eventual Destroyer
You can mix something with a Sony earbud, but it will be much easier with a decent monitor.
Although the earbuds won't suffer from the lack of room treatment, so they'd be good for comparison at least.
If two is going to cost you too much, you can get at least one and mix in mono, and rely on something less expensive for pan.
I second this.
One thing that I found helped the "portability" of mixes when I was stuck with crappy monitoring is: once you've bashed the mix roughly into shape, take a minute to drop a prametric eq on the two bus and play around with massive rips. Yank the high shelf down at the highest freq, then roll the frequency down and listen to how the mix changes. Do the same with the low shelf, rolling upwards. Yank the midband and roll it around, play with the bandwidth. Listen not to the effect on the overall sound, but how the relative balance of the instruments changes. This highlights the frequency ranges where the balance is wrong. When the relative balance remains more or less the same however you eq the two-buss, it will translate well to other speakers.
YMMV, but it worked well for me.