How do I know if my suspension springs are worn out?

When the car sits lower on one side and makes noise over potholes, there's a good chance it's the spring, especially on a 2008 307 that's already got some mileage on it. The thing is, it could just be the spring, could be the shock absorber too, or even the sway bar - that's why it's really important to take it to a mechanic so he can inspect it with the car lifted and figure out what's actually going on. The cost varies a lot depending on whether you're just replacing the spring or the shock absorber as well, but it's expensive enough that you'll want to get it checked urgently before it damages something else.

If the car is visually lower on one side, try this before going to the mechanic: park on level ground and compare the height between the front and rear wheel on both sides (measure with your hand, just to get an idea). Worn springs leave the car settling unevenly, and that's pretty obvious. As for prices, on a Peugeot that age you can find aftermarket springs for around 30-50€ per unit, but then you've got the mechanic's labor which varies quite a bit depending on the shop - it can range from 100€ to 250€+ depending on difficulty. I'd recommend calling two or three local mechanics with your car's reference and asking for a quick quote, because there are big differences.

Besides the signs I mentioned, pay attention if the car feels "mushy" when braking or turning, because a worn spring leaves the suspension without firmness. On a 307 with this age, if it's just the spring it'll cost somewhere between 300 to 600 reais per part plus labor, but the mechanic might find that the shock absorber is also worn out - then it gets more expensive. Take it in soon to get it checked because driving with a compromised spring damages the tires and the rest of the suspension.

A worn spring is pretty likely, but don't put all the blame on it yet - a blown shock absorber also causes noise over potholes and makes the car sway to one side. The thing is, on a 2008 307 they usually come as a package deal, so when one goes bad the other's probably not far behind. Take it to a mechanic, but make sure you tell him to check both components during the inspection, because fixing just the spring and leaving the shock absorber shot is money down the drain.