I use the specific gravity trick.
Remember, these fish deal with a lot of fluctuation so usually you can get away with being 'close' to what you want ( 1.010? I think pure saltwater is about 1.023)
I don't think table salt is a good idea, use the same salt used for saltwater tanks!
There's debate on whether the small amount of iodide or anti-caking stuff is bad but what is good are all of the trace minerals in sea salt.
hey sweets, one should not own a brackish tank if one does not know how to spell brackish :laugh:
just playing, but i agree with acesto, table salt doesn't contain all the necessicary elements that synthasized sea salt does, (though i've heard it can work for healing fish)