How to check if azobisisobutyronitrile (AIBN) is still active?
If azobisisobutyronitrile (AIBN) underwent homolytic cleavage over time, do you expect the resulting cyanoprop-2-yl radicals would survive as free radicals for a long period of time? If not, what would be the decomposed products?

Alkali metals reduce benzophenone in THF to form blue diphenylketyl radical anions. I'm not sure azobisisobutyronitrile (AIBN) or cyanoprop-2-yl radical can do that.


Maybe you can test the structural integrity of AIBN by 1H-NMR, IR or Raman spectroscopy?
The -N=N- stretching vibration of a symmetrical trans-azo compound like AIBN is "IR-forbidden" but absorbs in 1576cm-1 region of the Raman spectrum (1). If it decomposes, perhaps you can see the decomposed products in the mass spectrum.
Maybe you can try to activate the subject AIBN as a radical initiator?

a. Perhaps one can observe a minute amount of nitrogen gas upon homolytic cleavage?

b. AIBN can be used as radical initiator in radical polymerization. Once activated as cyanoprop-2-yl radical, it readily attacks olefinic double bond of vinyl monomers. Perhaps you can test such reaction and see if some polymerization occurs using mass spectroscopy?


Reference:


R.M. Silverstein and F.X. Webster: Spectrometric Identification of Organic Compounds (6th ed.)