How do I determine if a process is reversible without numeric information?
Any process is irreversible if it involves:


Friction
Heat transfer across a finite temperature difference
Expansion when there is a finite pressure difference
Mixing of difference substances under any real circumstances
Anything else where work is not being maximized


So air friction against the balloon rising at a non-zero velocity makes the process irreversible.

Heat transfer from the balloon when the inside temperature is different from the outside temperature makes the process irreversible.

For more information see: http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node34.html