I have started playing with friends online, and both of my spawns were horrible even with start bias turned on (hill tundra and plain desert). How do I know if my spawn spot is so bad it's not even worth bothering ? I'd rather not stay in a four hour game where I have exactly 0 chance of winning.
It's kind of difficult to give a hard rule here. However, if I was pressed for a rule of thumb, I would say that if your capital city and the tiles immediately surrounding it are more than half tiles that give no base food (ie, desert, ice, mountain, snow), then you're probably in trouble. In fact, without a balance of food and production, you're probably going to be at a disadvantage, so some forest or hills would be good as well.
In this case, you can move your initial settler to a slightly better tile. If you can do this within, say, a turn, you're probably not too bad off. If it takes longer than that or the surrounding area is just completely barren, you're probably at a significant disadvantage.
If you're in a slightly bad location, it probably just means that you need to found a second city earlier than you normally would. Usually your capital will be a powerhouse, but if you spawn in a poor location it might not be.
You can avoid this situation altogether by playing on more balanced map types - there are some that are completely devoid of desert and ice, for instance. Disabling start bias is also an option if you really want to play as a civ that usually starts near vast swaths of desert or ice.