This may well be the best £2 I've spent all year - quite simply every home should have a Draper Battery Tester. This unassuming little blue box has literally saved me pounds by preventing me from throwing away batteries needlessly as I used to.
It's easy to use, the top part lifts up to form an arm which can means that you can fit different sized batteries (AAA, AA, C, D, PP3 and 3R12) between the two contacts which are labelled clearly with +ve and -ve. Once you place a battery the correct way around between them, right way up, the needle will tell you in an unequivocable manner if the battery is good, low, or needs to be replaced, it really is as simple as that. It's child's play to read the display and, as far as I can tell the device is pretty accurate. I find it's surprising how often batteries that seem to have no more life in them that have been used for a child's toy or a remote control are actually perfectly alright for use in an item requiring less power. Often I might find that out of four batteries only one really requires discarding for recycling and the others can be redeployed, I use the tester most often with AA batteries as we seem to have a silly amount of items in the house which require them. The tester is safe and extremely easy to use.
I will grant you that the battery tester isn't exactly going to win any design awards on its looks, but it serves me well, and at 52 mm x 55 mm x 22 mm, it's small enough to stow in the battery box. It has a one year guarantee but frankly there's not really anything to go wrong and, with the price of batteries, it probably paid for itself in no time at all. It's fairly robustly made and seems to be widely available from DIY stores and online. Highly recommended, this little 50g item is great for the environment and for your pocket too, I really couldn't do without mine!