Time and practice. As you keep working on your dream recall, you'll be able to made distinctions a bit more easily.

Make sure you don't wait to write down your dreams. If you need to lie still for a little while to go through what just happened, do so, but then grab your journal and put it down on paper. You'll probably also find that the more you write down, the more likely you'll be to think "Did I... no, that was yesterday... no, that was last week... yah, that happened." Then again, you might find things repeating themselves , so if you think something happened again it's possible.

If you think something might have happened in last night's dream, write it down even if you're a bit dubious. As long as you have records, you can go back and see if that happened in another dream. You wouldn't want to not put something down because you weren't sure.