Point it towards the sun and then connect a voltmeter to the positive and negative terminals and it will give you a reading. The output is going to be more on a bright sunny day than that on a dull day so make sure you check it in bright sunlight to achieve accurate results.
There should be some information on a plate on the panel but it does depend on how small the panel is.
Most panels work at anything from 17v to 21v, some work at 30v so check the voltage against the plate and use a ammeter to check the amps with the panel facing the sun
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.