If you have access to native Sabre, you can display the arrival/departure taxes for each country that you're traveling from/to (TX*followed by the country code, e.g., TX*US for USA, TX*GB for the United Kingdom, TX*DE for Germany, etc.).

You could also go onto the other airline's website and price the itinerary. Once you've selected a fare (commerical fare) you should be able to see a breakdown of the base fare and the taxes (sometimes even a breakdown of taxes). There are a handful of taxes that are a percentage of the base fare (e.g., the US DOMESTIC TRANSPORTATINO TAX which is 7.5% of the base fare) but most are a flat rate. That should give you a ROUGH idea of what the taxes will be.