First of all, let the steering wheel go at 60-80 km/h on an empty, straight road - if the car pulls to one side or "weaves," then the problem is definitely in the rack. If it drives straight, then the play is most likely in the tie rods or bushings, not in the rack itself. This is the most reliable test because the rack affects straight-line handling.
Next you'll need a pit or a lift. Position the car so you can see the steering linkage. Have someone turn the wheel left and right with a 5-10 cm range of motion while you watch the joints. If you see play in the tie rods themselves (they start rattling in their sockets) - replace the tie rods. If the steering linkage moves as one unit but all the "play" is felt in the steering column or somewhere deeper under the hood, then it's the rack.
The catch: don't confuse play in the rack itself with play in the rack mounting to the body. The unit can be fine, but if the mounting bolts have come loose, the wheel will still move around. So before replacing the rack, always check that the bolts are properly tightened. On your sports car this doesn't require any special skills - just look under the hood around the bulkhead. If the wheel visibly moves along with the whole assembly, then it's most likely the mounting, not the rack itself.