Hello Andrew,
Blue-point do a Combustion leak detector kit .
You fit a tube/cup container in place of the rad or header cap and in the cup/tube a fluid is poured and if any combustion gases enter the coolant the fluid changes colour, different colour for petrol engine as to diesel engine.

I would guess this kit will be available by other supliers but ask if it will do petrol and diesl engines.

You could also use a coolant pressure tester and see what the pressure gauge reads like. I.e does the needle bounce up and down as the pistons pressureises the cooling system

HTH,
Ian