You can safely assume that the first one (as long as it is really early, pre-200) is ALWAYS better as an Academy. After that, just do a calculation. When you get it, just hover over the "Research" command button that tells you how many Science points it would generate; then go to your science city, and hover over the Science points to display components, and look for the general modifier (in %) applied to the Science points. For example, if your academies are making 10 points each, and your general science modifier for that city is 83%, you would be generating some 18 science points from a new academy from now on. Compare that to the research value.
For example, you get a GS with a research value of 2287. Your science city has a modifier of 83%, and your academies are generating 10 points each. Thus a new academy will generate 18 science points after the modifier; it needs therefore 2287/18 ~ 127 turns to break even with the research option. If you are now in turn 200, it means that given present conditions, from turn 327 and on you will get more from the academy than researching now.
Of course, this is only a good baseline. Other considerations are also important; if you can get a critical tech, perhaps with a critical wonder ahead of the rest, then that adds to the value of the research option. And so on...