Look at the cut end of the wood. If the wood has a green tinge, it is probably pressure treated. Over time, the wood will wear down and no longer have this green tinge, so you can cut a small piece of wood. If the inside is yellow or white, the wood is probably not pressure treated.

Is there a problem using it for holding up tomatoes?

If you found it lying around in the garden and it is not showing any rot, it looks solid in the picture, then it is probably pressure treated. Pressure treated wood fresh out of the sawmill is probably not advisable for use against vegetables, but as your post is much older than this then it will not harm anything.

I have to say I have never heard of anyone suffering through proping up their vegetables with a piece of wood, pressure treated or otherwise. Does anyone know the statistics?

My fences have obvious tinges of green on them so they I can identify them as PTW, but this plank of wood has no green tinges on it. Furthermore when I cut the wood the inside of it was white which indicates more that it is not PTW.

The tests would say the wood is safe thereby indicating my tomatoes are safe too. I want to eat my tomatoes but I also don't want to risk eating arsenic. What to do?

Take a look at this.http://www.thesurvivalistblog.net/is-treated-lumber-safe-for-building-raised-bed-vegetable-gardens/

The amount of arsenic transfer from old PTW would be negligible and far less of a hazard than most of the chemicals that are probably using to control pests and diseases - probably even less than the poisons that occur naturally in any of the solanum family of plants.

Re your second question, it is probably pine, hemlock or fir or any soft white wood!

Arsenic has not been used in pressure treating wood for decades.

I have since eaten a few of my tomatoes which were supported by the wooden stake in the picture. I haven't had any immediate side-effects from eating the tomatoes, so I'm just hoping it stays like that.

Thank you everyone for your well-advised input.