I personally never grow tomatoes. I'm collecting some seedlings that grow wildly from tomatoes that people throwed outside. I couldn't figure out whether that small plant on the right is a tomato or not?
The plant on the left side I'm sure it is.
There are two main things, based on which you will be able to recognize a young tomato plant:
1) The stem
It has a little strings like hair all over it, which are basically potential roots. When they meet dirt, they become ones. Every tomato plant should have them.
2) The leaves.
In order to recognize whether they are from tomato, you have to see the picture above. As every plant, they are typisch and special.
I can not see the stem very well on your picture, but based on the leaves, it doesnt look to me as a tomato plant personally.