I look at cost of repair(at lbs) vs cost of purchase(at store). The sooner the cost of a repair begins to approach a significant part of the purchase price, the more likely it is that it's a BSO.
A "rea"l bike should make financial sense to repair, beyond flats and brake/derailer tuning.
Take a department/web store bike for 100-something USD, not really a lot of repairs that you can reasonably have done on one of those.
Heck, even the cost of having tubes+tires replaced can easily be 1/3-1/2 the cost of purchase.
Or let's say that the crummy sus fork begins to squeal intolerably, and you can't stand to wait until it's rusted solid.
Even with the cheapest noname replacement fork Ebay have to offer, with work, you're still looking at half the purchase price or more to have it replaced.