Lorann's will list next to the flavor if it is an oil. You can also check the ingredients for each of the flavors on their website. As someone else also mentioned, there are some quick charts which show which flavors are not soluable in water.
FWIW, many of us have experimented with the natural oil flavorings. Something to consider, is that you won't use them in the same concentration that you would say, Strawberry, or Caramel (which do not contain oils). They are in a much higher concentration. If you were to use 20% of that cinnamon oil you're gonna get knocked on your keister!
With anything DIY, you do it at your own risk to yourself and your hardware, but many of us have used the oils (sparingly) and have had no ill effects to health or to our hardware.
I hope this helps.