I presume you are refering to organic search? Because for paid search this would be fairly easy - you could just set up a little test campaign.
With regards to organic, it depends a lot on what kind of SERPs is displayed. Having Adwords on top, images, Map, places, knowledge box etc pushes the organic further and further below and CTR fails. On the other hand, if the SERPs does not display any of the above, then CTR is greater, but in both cases it further depends on the position in SERPs.
You can determine CTR opportunity by using google webmaster tool data. Download list of keywords from GWT To do that go into Search analytics Check Clicks, Impression, CTR, Position Once you click on all these parameter download the file. Sort the file from CTR column You will have very good idea about the CTR you will get on what position in your niche.
You can take idea from google webmaster tool. You can find search term, impressions, clicks and CTR as well in google webmaster. From their you can select your keywords. And you can decide which keywords is profitable for your business and you can target it.