I do it the way derek uk suggests.

Something I haven't seen mentioned yet - Volvo incorporated a service socket to crank the starter without activating fuel or ignition systems. (Unless the car has a ballast resistor in the coil supply line, in which case disconnect the coil #1 terminal)

On my 83 it is located behind and above the cyl head protruding from the harness on the firewall there. It's a Blue wire, usually with the final few cm's wrapped in a Red label sleeve. Connecting its female spade terminal to +12V cranks the starter. With a simple setup of wire and temporary contact pushbutton switch you can crank the engine from under the hood while watching the compression test gauge.