You may want to look up the process known as "LDAR" (Leak Detection and Repair) which is an inspection of pipeline components which is mandatory in US refineries and similar sites, and essentially involves "sniffing" of valves, pumps, flanges, open ended lines etc. using a flame ionisation detector to look for VOC fugitive emissions. (It is the light ends that leak more thn the heavier compounds of course.) Obviously this only applies to your hydrocarbon lines, not water, and glycol whilst theoretically being a VOC is not that easy to detect (though gas dissolved in glycol streams does show up in joints that are giving emissions).
Having done a lot of LDAR survey work myself, I would say that typically large joints are more likely to leak than small ones, though there are less of them, and things subjected to thermal cycling and perhaps vibration (suction / discharge lines on compressors for instance) tend to be more likely to be found leaking. A good LDAR database can be invaluable on-site as might show repeat leakers, help grade gasket and packing suppliers and the fitters as well perhaps.
Corroded components are common on coastal gas plants but should be easy to spot, and as has been said already if it isn't broke then don't try to fix it, b ut be wary of pressure containing parts that might be badly corroded of course. However, if some emissions are modest (i.e. valve glands that can be nipped up) then sometimes an early first-fix can be useful, and save larger leaks from developing. However, do be careful with flanges having compressed fibre joints as these can embrittle and could crack if disturbed.