It is normal to not be able to hear the turbos on these cars. If you want to check the turbo remove the U tube between the air filter and the turbo compressor inlet (engine off). The turbo wheel should spin freely and have barely any play side to side or in and out. If you really want to make sure the turbo is working there is a line running from the back of the intake manifold to a switch on the firewall, then to the ALDA on the injection pump. Disconnect this line (and clean all the black goo from it while you are at it, search for "cleaning banjo bolt"). Test drive the car with the line disconnected. Low on power? The turbo is working fine.

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine