a meter does help, without using a meter to verify the panel’s output you are just guessing. Even cheap ten dollar meter would work as you are dealing with low voltage and really do not need the protection of a Fluke meter that is fused to protect the user from accidental electrocution
nearly all solar panels will have a label on the backside stating voltage , watts and amperage … or if only a model number just Google the model number with manufacturer’s name…
Also note many solar panels carry unbelievably long warranties… several manufacturers have 25 year warranty periods