I believe shell suits should have a crinkly polyester outer layer, whereas tracksuits are more fabric-y (not a real word).
I remember as teenagers we had a bonfire night camping party (as dangerous an event as it sounds) where a friends shellsuit came close to spontaneously combusting from sitting too close to the fire. Carrying on this form, he proceeded to heat a large steel rod in the fire, went to pretend to throw it in a javelin-esque fashion, only to drop it due to the heat where it proceeded to slice through a friends tent, which only managed not to burst into its flames due to its (thankfully) flame-retardant fabric. Luckily no one was inside at the time...
Good times (?).