This is a sister to this question.
I need a method or a trick to measure (or guess) how saline is my food using normal tools or substances available on the super-market
Here's something useful to know. A good typical salt level for most savory foods is in the neighborhood of 1% by weight. So if you know how much your dish weighs in grams, then add 1% of that in salt and you should be very close to a good result. Adjust down if you are using any ingredients that are already salty, like capers. Adjust up if after the first few times you try this, it is a little light for you. You will also learn how much your salt weighs by volume so soon you can just use a measuring spoon or a pinch instead of weighing the salt.
The normal test for salinity (using a hydrometer, which you're not going to find in a supermarket) won't work, because you have a mixture that's more than just water and salt -- soup will have lots of other things in the water (sugars, gelatin) so you can't take a single measurement and determine salinity from it. You can get one from a pet store -- they're used for maintaining salt water fish tanks.
The other common test is to measure how well the sample conducts electricity ... I don't think this test would be affected by other compounds, but it's going to measure total salts, not necessarily the sodium level.
If you want just the sodium level, you're going to have to stick with chromography, which ... you're not going to find in a super market, and likely much more expensive than what you're looking for. (and as they use salt slides, I don't know know that might affect the tests)