The cheapest method is to have a wrap-on repair kit available and wait until the pipe useful life in the affected areas is complete because it leaks. Some facilities use an external, blind-drilled hole in the most erosion-susceptible location - leakage from the hole shows that the pipe thickness has decreased to that selected thickness equal to the hole depth.
More sophisticated techniques range from the semi-automated UT thickness testing from inside the tube already suggested, to IR inspection, to discrete manual thickness testing with UT instrumentation, to laser internal inspection (similar in concept to IRIS, but using a laser to measure the ID) and radiography.
