Help with guppies

Here's my question, How can I find out the sex of my two guppies?

So as it turns out after buying 22 guppies two weeks ago for my piranhas to eat, 2 have survived and they have been swimming together ever since.

I wonder have they "paired up" and their love has strengthened their will to live? Or am I dealing with a couple of fish that just stuck together to survive.

At any rate I removed them from the P tank and gave them a 10 gal for now.

If I can get them breeding I would love to experiment with that for fun and/or practicality.

Maybe they are too small still, and they don't have any color either like the one in that pic.