You can sometimes determine that by looking for heating duct runs (if you have them), return air grilles, determining which walls in your place are bearing walls, and if you have hardwood floors, the strips s/b running perpindicular to the joists (but they don't always...). There's a lot of things that you can look for.
Standard residential construction code calls for a 10lb/sq. ft. dead load rating (that's the stuff that the floor is made of) plus a 40lb./sq. ft. live load rating (that's what you put on the floor) (so that's a total of 50lbs/sq. ft.) in the middle of any floor span. - and that's the minumum. We tend to put our tanks close to a wall, and we're able to put higher loads on those areas. I would not put that size of a tank parrallel to the joists, so it's important that you determine which way they're running.
Or you could stick with your 60g tank replacement idea. :smile:

Mitch