How can I set the drag on my spinning reel with out a scale?

One gallon of water weighs 8.3 pounds you can figure it form there

2 Cups = 1 Pint = 16 Ounces

2 Pints = 1 Quart = 32 Ounces

4 Quarts = 1 Gallon = 128 Ounces

Practice.  Know your spinning reel, how it handles, what kind and the size of fish you're targeting, what size line, etc.  Then, you can pull down on the line at the end of the rod, estimating how much you think a good sized fish you are targeting will pull (without breaking it) and then adjust accordingly.

I just pull the line and know from experience what drag I want. I also know that it's approximately 20-25% of the line's rating.