how can you determine wether a new set of wheels will throw off your speedo?

for instance: going from a 15" to a 16" setup....what would you do to try and keep things on track?

 

I'd say if you are going to 16's then the tyre profile will be smaller and you might end up having the same wheel dimensions as with the 15's. But you would have to calculate it and see how close the change would be.