One of the recurring questions on GIS stackexchange is “I have these points with an unknown projection, can you help?” (at last count more than 100). The answer always depends on if the hapless user knows roughly where they should be. Hint: if you don’t know where your data should be or it’s projection then you have a list of numbers not a spatial data set! The next suggestion is to ask the person or organisation that supplied it (sadly this rarely seems to help).
So to help out with this (apparently) common issue I wrote some GeoTools based code to attempt to find a matching projection.
First it looks up a location using the GeoNames API to get a target point in WGS84. Then we can hunt through the CRS list and pick any which are in the area of validity. Finally we can try transforming the WGS84 point using these possible projections and keep the one which is closest to our unknown point. One small wrinkle is the need to convert the distance to metres (otherwise the ones in feet are always 3 times further away than expected).