There really is no way to tell without examining the structure. There are way too many factors, such as:
1. Joist Size
2. Joist Spacing
3. Joist Material
4. Subflooring
5. Distance from load bearing wall
6. Additional span or point loads
7. Damage caused by wood rot, termites, etc.
8. Quality of workmanship and material
Now, with all that said, if you are planning to install it, I would reccomend you:
1. Install it on an exterior wall (shear force placed on a joist increases and the load is moved further from the point of support, up to a maximum load equidistant from each point of support).
2. Determine, if at all possible, the location of the joists and span as many as possible to decrease the load placed on any one joist.
3. It would be my reccomendation to set it up slowly. Do not fill it all the way up in one day. I would add maybe 20 gallons a day for 5 days at the fastest, just incase something does occur, you can catch it before any major damage. It would help to know the people below you.
Some sort of disclaimer: I personally think that when you exceed a 50 gallon aquarium, you are far better off placing it on a concrete surface, such as a lower level. With the typical freshwater aquarium loaded weighing in at 12#/gal, you can cause structural damage if the tank is not properly supported. If you must place it on an elevated, wood constructed floor, take due care and caution to avoid damage to the surface. Before installing your aquarium, inspect the location you plan to install it.