The way I tell a cheap polish is by its smell. These smell revolting, so its actually hard to me to sit down and use these.
They also don't dry very quickly. Can't be doing with that!! No time to just sit around with wet nails!
They DO have some fantastic colours tho. That is the funnest thing about them for me. Fortunately, I didn't buy the 6 I have, it was a gift. I'd be really bummed if I had paid with my own money.
I purchased a few polishes from Kmart for $3 and decision wise it is quite hard to narrow down what colour to buy coz of the large colour selection.
That being said, it isn't the greatest quality polish though considering the price it's not really surprising. On application, the colours are a bit streaky and not greatly pigmented, they're quite sheer even after 3 coats. I also found they chipped quite easily and began to crack as well which isn't the greatest thing in the world.
If you're after something to wear for just a day or two, then go for it, but if you want a polish with a little bit more longevity than I wouldn't definitely splurge on something a tad more pricey.