At this web site

http://www.kester.com/en-us/document...de%20Line).pdf

Kester gives a test for whether solder paste is fresh enough. The test is
to melt a drop of it onto an unsolderable surface and see whether it forms
one ball or several. If it sheds a number of smaller balls (rather than
forming just one), the flux has deteriorated. The failure mode is flux
deterioration.

The funny thing is, I have some solder paste that is 10 years old and seems
to pass this test. I'm going to repeat the test more rigorously.

Presumably, paste that is too old for surface mount work could still come in
handy in larger-scale work. It's still solder, after all.