There will be a voltage rating on a label, assuming it hasn't been removed, that will state the VOC (voltage, open circuit), and you should get somewhere near that voltage in full sunlight.
Also, to check current, printed on the label as ISC (current, short circuit) you simply short the leads of your multimeter (in amps mode) across the leads of the panel. If the panel puts out 10 amps or less, then most normal digital meters will handle that. If the panel is rated at above 10 amps ISC then you will need either a shunt or an amp meter that will handle that current.
You will see somewhere around or maybe a bit less than the rating....this is normal.
If you get less than say, 80% of the rated voltage or current then that panel probably has some bad cells.