No matter what the water leak is on you have three options.
Reasons you might leave the leak:
If the leak has "crusted" over.
Its not going to cause any damage or harm.
Its not getting/going to get worse.
If its not on show.
This is a common course as its the easiest and cheapest but we recommend you repair or replace.
Times you can repair:
If its on a compression joint it may just need you to tighten it up. Or if its the olive at fault you can wrap some p.t.f.e tape around it, but will require you to drain down.
Replace the washer if damaged (won't stop no matter how tight you try to get it).
"Repacking" of certain spindle type valves like radiator valves or gate valves. (Leaking from the spindle/middle.)
With soldered fittings you will need to drain down, so there is no water in the pipework whatsoever. Unsolder old fitting, clean pipework at joint to be remade properly and remake with a new clean fitting.
Holes in pipes from corrosion or nails are remedied by renewing that section.
Time to replace:
Cracked or split bodies of pumps, valves, radiators or heat exchangers.
Old out dated fittings or components.
Or when its cheaper and easier.
Locating leaks are relatively easy as long as you can see or touch the area your checking.
Firstly damp patches on flooring or work surfaces, dependant on where the leak is, are a quick and easy way of locating a leak.
Secondly you need to keep an eye out for damp, mould or other water marks or rings on floors, walls or even ceilings.
Thirdly clumps of "crust" around pipework or fittings is another indicator of where the leak is. Although the crust is usually a sign that it has sealed itself at this point.
If the leak is not too severe then adding a leak sealer to the system is always an option for you (not for systems where the heating water and potable hot water can mix, like primatic cylinders - gravity systems).