Sometimes there may be an external sticker that indicates what the capacity/capability of the panel might be .. but these dont last long. If you are able to get to the underside and have a look around the small junction box where the power leads leave and head towards the 'battery', you might find some indication there .. there are ways to work out the wattage .. but knowing the quality/condition of the receiving battery and having a sunny day is also essential ..
If it still seems confusing, just pop into a 'solar' agent somewhere for a firm answer. These days, there are many and varied sizes .. some have the same wattage capacity but can easily be quite different in size .. (mainly due to later technology ) ..
I'm not an expert in these fields either, and I do usually go to the appropriate sources to help me out ..
