A lot of my photo's seem to have incorrect white balance, but as I shoot in raw I'm thinking this can be corrected. So my question is, with post processing, what's the best way to ensure correct white balance?
Grey Card or Neutral Object in Shot
Shoot with a gray/white card in one of your images, or select something you know to be neutral (white shirt, pavement/concrete, white wall). Most photo editors will have a white balance dropper tool. Click on the neutral area to adjust the white balance. If you then note the color temperature selected, you can sync that to your other images.
Remove Color Cast
If you have nothing gray, you can use the following trick.
Strange as it sounds, this technique works very well. Obviously if you have a macro shot of blueberries, it will remove a lot of blue, but for most images I've used it on, it works amazingly well. Even if it overdoes the correction, it tends to be in the right direction, so you can use the layer opacity to suit.
I use the above techniques to estimate what the "correct" white balance should be, but then I almost always adjust it to what looks good to my eye. "Correct" white balance may suck all the warmth out of an image for example, something you may not want.
RAW files do not have WB applied except for the embedded JPEG used for preview by software that do not understand those files. So, it is up to you to apply the correct WB when converting those files into images. There is no correction to apply because nothing has been set.
Just to be clear, there is no WB setting which will correct all your images. Each set of images shot under the same light though should require the same WB to be set. If you use an application which allows to synchronize settings between images such as Lightroom, you will be able to set the WB once per set and then synchronize WB.
The ideal situation is that you take an image with a WB target each time you take photos under a different light. In this case, you first open that image and use the WB tool of your software to pick that as the white-point. It should turn white and colors should appear neutral.
Should you have forgotten to do that. What you need is to do the same with an object that you think should be neutral, a piece of paper in the scene, a white t-shirt, concrete, etc. Anything that you know is neutral in color.
Without any such thing, you have to do things by eye and that requires a well calibrated monitor, otherwise you can make things worse. For example, if your monitor is too yellow when you make things look neutral, they will end up blue. What helps in a scene with no neutral objects is that viewers also lack a reference point, so they will be less prone to seeing a color-cast.