Just started ageing, how do I know if meat is going bad

[Ron Pratt]

No telling how long either of those pieces had been in a display case, even though one was a piece you already had. That is why we always recommend dry aging a sub-primal purchased in a sealed cryovac bag and then transferred to the UMAi Dry® bag using the clean transfer method. Also you should NEVER place 2 pieces of meat in one bag as the hidden butt ends or sides depending on how you placed them in the bag are ripe for ruining the meat.

At this point your best best is to open the bag, smell it closely and either consume or pitch it, but my guess it is already rancid.
Ron