A big leak will hiss, usually loud enough for you to hear it if you take a piece of 1/4" ID hose and use it like a stethoscope. You can also tell if a rubber hose is leaking by pulling it off and putting your finger over the fitting -- if the idle changes, the hose was leaking.
The rubber connectors must fit tightly enough that the hose won't slide out, and if they are no longer pliable, they leak. Plan on replacing all of them. If the rubber parts rattles around on the barb (this especially applies to the idle control valve), it is leaking since it isn't sqeezing down any more.
Leaking actuators, transmission modulators, etc must be checked with a MitiVac hand pump, about $30 at any auto parts store. This is a hand gadget with a hand powered vacuum pump, guage, and a bundle of adapters. It will pull, slowly, a petty good vacuum on any system on the car. Some have tanks, and will take a while, but the MitiVac will get them down to 15" or so mercury with some pumping. If you cannot, for instance, get the climate control to hold vac by pumping at the green line in the engine compartment (disconnect from the engine and connect the mitivac instead), you have a leak and will have to check inside the car.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!