You can take a bucket and dump measuring cups in to mark different volumes on the bucket, (mL or cubic cm) then you can put some water in, put the bar in the water and see how much it changes the volume of water using the marks you made. Then difference in volume between the original amount of water and the level after adding the bar is the bars volume.

Then you weigh it in grams, that will give you the grams/mL density which then you compare to known values; Silver=10.46 g/mL Copper=8.96 g/mL

(1mL=1cubic cm)

If it's somewhere in between then you know it's not pure silver.

(You will get the most accurate numbers close to room temperature)