Good way to get started is with a compression check, you want it to be within 10% of each other across all cylinders. Haynes manual should have specifications for the GSR for what the compression should be and you should be able to rent one at autozone. Engine should be warmed up, ecu fuse should be pulled, and throttle plate should be wide open during cranking, for about 10 seconds.
Second thing i would consider is having your head sent out to get new valve seals, pressure checked, and decked, then install a new headgasket and timing belt and water pump. These are all precautionary measures given the age of the engine.
You realistically need to make sure that theres no signs of knock, and that the rings are not worn to the point where they cant handle the boost pressure. Compression test should tell you the condition of the rings. Leak down test if needed.
