You try to turn it and the handle won't budge, or it snaps off in your hand. Gate valves seize after years of not being used, especially in hard water areas like Swindon. If you can't turn it, you can't isolate the water in an emergency, and that's dangerous.
Water drips from the top of the valve where the handle spindle goes through. This is usually because the gland packing inside has perished. You can sometimes tighten the gland nut to stop it, but if it's old, it's better to replace the whole valve.
You turn it fully clockwise (closed) but water still dribbles through. The gate inside is worn or corroded and doesn't seal properly. This is common in old gate valves. You can't isolate the water properly, so it needs replacing.
The handle turns but you can still hear water flowing. The internal mechanism has failed (spindle broken, gate detached, or threads stripped). The valve's useless and needs replacing immediately.
Green corrosion (verdigris) on copper pipes around the stop tap means it's been leaking for a while. Even if it's not actively dripping now, it's corroded inside and will fail soon. Replace it before it becomes an emergency.