Take the heaviest sandbag you can handle (preferably without handles) and lift it from ground-to-overhead for 20 reps. If you get the loading right you should start to struggle around 5 or 6 reps and probably end up dumping the bag after some unsuccessful reps later on in the set.
Record the weight of your bag and the time it takes you to complete the 20 reps.
Men and women should aim for a bodyweight sandbag.
Train hard!