Is she motorised? if so try her on a calm day and see how she maneovers, make 2 or 3 sharp turns to see how far she heels over then do a righting test ( pull her over from the top or the mast until the water just touches the bottom of the hatch comming . Then let go, if she springs upright she should be OK to sail in light to moderate weather if she hangs over or only rights slowly, she needs a ballast keel --- If you put a keel on you will also probably need to increase the rudder area or she won't steer
It is really a case of suck it and see. with out seeing the model All I can do is give general advice . Just remember this most model barges are sailed in the Light ship (no cargo)position but it is quite acceptable to sail in a loaded position which allows you more ballast
Jimmy