How can I check the sodium content of food?
Sodium is a difficult ion to estimate at home. 

Since most sodium salts are water-soluble, colorimetric or spectroscopic methods are often employed, both of which I assume are not within arms' reach.

Some alternatives:


Speaking of sodium estimation in foodstuff, it is usually determined by Mohr's
method which involves the precipitation titration of $\ce{Cl-}$ ions. This is done under the assumption that sodium is mainly present as sodium chloride. Additionally, it will also register $\ce{Cl-}$ from other chlorides, and hence may give an elevated reading. You might be able to do this at home. Here is a page
to help you with the process.

It is done in the presence of chromate ion ($\ce{CrO4^2-}$), and $\ce{Ag+}$ ions are added gradually. This precipitation happens initially:

$$\ce{Ag+ + Cl- &lt;=&gt; AgCl(s)}$$

When the solution runs out of $\ce{Cl-}$ ions, precipitation of the red-brown $\ce{Ag2CrO4}$ begins:

$$\ce{Ag+ + CrO4^2- &lt;=&gt; Ag2CrO4(s)}$$

This signals the end point.
There are sodium ion meters, you can get them for ~300$.
There is a volumetric estimation[1] for sodium directly, but it is not expedient to do it. It is based on the precipitation of sodium zinc uranyl acetate.


Conclusions

IMO it is difficult to do this as a home project, as it will require stuff that is not usually found at home to do it precisely.



References

[1] J. Am. Chem. Soc. 1931,  53, 9, 3288-3291 DOI: 10.1021/ja01360a011