Every time you approach a mooring you should be asking the "Is it safe to use ?".
When approaching a mooring that you do not own and maintain - how can you be sure whether mooring (the object on the sea bed, the chain and the connecting hackles) is in serviceable condition. I've seen shackles with the pin so corroded that it remains only loosely in place by virtue of the seizing wire holding it in position - the pins threads long size have lost contact with the shackle's body.
If a mooring maintained b y a harbour authority who except that they will keep them in good order, back it up with your own assessment of the condition of the parts you can see, and be wary of any marked "Use at own risk" - what are they trying to protect themselves against.
Don't exceed the marked weight for the mooring. These may be marked 'Max 5 tons' or 'Max 10 tons'. If you are on a harbour/local authority 10 ton mooring and your boat weighs 5 tons - don't be surprised to find another yacht raft up alongside you and take a bowline to the same mooring. Ensure well fendered, and that the spreaders of the two boats are staggered to avoid rig damage.
If the mooring doesn't have a maximum weight marked on it, and your are uncertain what size vessel it is intended for - enquire. In Brittany I watched 4 cruising yachts pick up a small light mooring. The wind picked up, and you've guessed it, the whole lot went walkabout with the yachts only narrowing averting a drift onto rocks - as the crew of one vessel were alert enough to fire up the engine and motor the raft away from the danger. Apart from putting their own vessels at risk they also managed to wreck the poor owner's mooring.
When picking up a mooring enquire whether you are free to do so. Aside from "what weight of boat is it designed for", "is it in serviceable condition" you should ask "Will the owner mind/will he be returning ?".
Bear in mind that moorings marked on charts or in guide books may only be seasonal moorings, and may not be present during the winter period. But you do carry an anchor onboard don't you ? See anchoring techniques.