My favourite matches are the really close ones, where both teams are in bad shape, and inevitably you will take tower and inhibitor losses there. In my experience, there's a simple checklist for whether or not it's hopeless.
1. How many dead lanes (towerless) do you have? Usually they'll all be worn down at a similar rate, but if one or two are untouched, or even one has one turret remaining, that's a buffer that gives you reaction time, prevents the enemy from coming from there, and generally makes defence easier. I'll always feel more confident if there's some kind of buffer, even if it usually gets torn down in short order - that's time to organise, kill, maybe get in a push.
2. How many inhibitor towers are down, and what are the active ones like? Same principle as for dead lanes - it's additional reaction time. This is always the key to turning a match around - having as much time as possible to do it in. Just one down can happen even in a match you're winning, so isn't a huge worry. All three is cause for concern, as some champs have enough damage to chain between them before you can properly react.
3. How many inhibitors are down? One can be handled by a lone carry, though it'll force you to push along that lane or leave someone in defence. Two is harder, and I'd suggest consistent defence, maybe trying to BD an enemy turret to keep their attention split. Three is virtually impossible to defend against - it requires all the attention of every team member to just hold out. Coping with hostile champs as well means that the slightest mistake can result in defeat. The exception to this is...
4. How is their base? Even knocking out one hostile inhibitor is an enormous boon - one tends to think of it as an offensive tool, but if you've lost inhibitors, spawning supers is an excellent defensive weapon. It'll effectively cancel out one of your own lost inhibitors. I've even been in matches where all six inhibitors are dead.
5. How good at they at killing? If they have an overfed Yi or similar who can hit you hard while trying to defend a tower, or even if they are simply coordinated as a team, it's a bad sign. Learn who to focus down, don't take risks, and learn when defence is hopeless - don't bother running to a tower being hit by Yi if it's on low health, as he'll probably kill it and you.