Yes, a float test should work. However, 5 hours may not give the best results. Seeds that sink immediately or within the first 12 hours are very likely to be good. Seeds that sink after 12 to 24 hours are still somewhat likely to be good. I would discard seeds that do not sink in 24 hours unless I just had plenty of room and wanted to take the small chance that they might do something.

I did as you suggested, actually - after 24 hours one was still floating. I weighed it and it weighed about half the others.

So, out of curiosity, I decided to cut it in half to see. The cotyledons were white, but almost hollow. It may or may not have sprouted.

I use "Float Test" when selecting seeds that I save from tomatoes. After fermentation, I pour them in a tall glass and stir. The best one sink right away. I pour most of the water and floating ones out. I do it few times until I have just the ones that sink right away. Those are the ones I save. This way , not that you will have an improved germination rate, but you select the BEST seeds that will grow most strong and healthy plants. That is how the nature does in the process of NATURAL SELECTION.