Measure the output into a good dummy load, using CW as the mode. Select CW, plug in a key, or a switch wired to act as a key, or just two wires connected to the proper type of plug. Key it with the power full up (assuming your dummy load is capable of dissipating the full output).
See if your output is at or near 100 watts.
That's your PEP output. Most meters just don't work fast enough to see the full 100 watts. A good reading might be 25-30 watts (25-30% of rated output power). Keep an eye on the ALC and don't overdrive things. You should NOT see 100 watts into the dummy load unless you're looking at things with an oscilloscope.