The only way to really know is to open it.
If the stuff on the top is hardened, It's done for.
A little different type of paint but for example.
I had a can of Silver Bullet rust protector that was 20+ years old.
I just knew it was not good anymore.
I had to almost chisel the lid off.
Once I stirred it up, I worked great.
With your primer,
I would probably use it all once I opened it.