You can do a wet and dry compression test or take it to a grarage who will be able to carry out a cylinder leakage test (measures the cylinders sealing ability)

To do the compression test, first measure the compressions and write down the readings for each cylinder, next drop a few squirts of engine oil down the plug holes. Do a second compression test and if the the results are substantially higher with the oil then the rings are knackered (the oil seals the rings/bores)