To measure across the solar panel terminals or wires put the red positive meter lead on one side and the black negative on the other.
Which wire on a solar panel is positive.
Grounding the positive is also nec compliant and would thus have a white wire carrying positive.
The striped cathode of the diode will be pointing towards the positive side of the solar panel.
White for negative denotes this is a grounded system and the negative is grounded.
Just like a typical battery you may be familiar with solar panels have positive and negative terminals.
A simple voltage reading will show you the polarity of a solar panel even when inside.
When solar panels are wired in parallel the positive terminal from one panel is connected to the positive terminal of another panel and the negative terminals of the two panels are connected together.
The positive wire has a female mc4 connector and the negative wire has a male mc4 connector that snap together forming a connection suitable for outdoor environments.
Red and black are acceptable colors to use.
The positive wires are connected to a positive connector within a combiner box and the negative wires are connected to the negative connector.
Solar panels are not dangerous if you know how to handle them safely and how they work.
If this is an ungrounded system also compliant in some circumstances then neither wire should be white.
At the other end of each wire is an mc4 connector designed to make wiring solar arrays much simpler and faster.
The truth is that they are not any more dangerous than the main panelboard that you have installed in your property.