I think these cheap
panels are becoming a great option. The price
has dropped dramatically and some of the big players are not reducing their prices accordingly.
Check the claimed output (preferably NOCT data if they provide it) and divide this by the area of the panel to get an idea of its efficiency. Really you can only tell the quality by looking at and measuring the panel. The rigidity thickness of aluminium framing and glass will give some idea of the mechanical durability.
It's easy to measure the output, but you really need a control panel
that is getting the same conditions to compare the panels. This is not as difficult as it sounds as the two panels can be very different sizes and still compared. A 100w panel should put out twice as much current
as a 50w panel for example.
My impression is the cheap
panels are a bit more optimistic with their claims so maybe deduct 10% output if comparing them against known brands. I also think the durability will be less, but even they only last 10-15 years there is likely to new solar technologies available.