To predict the sinking/floating behavior of a spherical object, you could determine its density. 
Step 1. Determine its mass on a suitable balance. 
Step 2. Determine its volume 
Either 
Measure its circumference. C = 2πr, so you can calculate r 
or 
Measure its diameter. d = 2r, so you can calculate r 
Then 
V = ⁴/₃πr³ , so you can calculate its volume.
Step 3. Calculate its density (D). 
Density = mass/volume 
If D < 1 g/cm³, the object will float in water. If D > 1 g/cm³, the object will sink.