Encyclopedias typically have their entries written by established scholars in the respective fields the entries are about (each entry should mention its author) - as such they are not a bad place to obtain some basic information. However, they are also extremely condensed. I've heard sources like dictionaries and encyclopedias being called "tertiary sources" many years ago. You would typically use them to get something like the working definition of, say, "despotism", if its not a main focus of your paper, or a quick digest of "who was Napoleon Bonaparte". In a sense, an encyclopedia is wikipedia without the anonymity, thus potentially making it the "better" alternative (although some wikipedia entries are really really good).

Effectively, at the bare minimum, a good secondary source does cite its own (primary and secondary) sources; a good rule of thumb is, if the source does not provide citations itself, don't cite it at the college/university level. Also, thanks to the internet, you can easily look up the author and see if he is a scholar writing something about his own field or not. Another criterion is where was the paper or book published in you are looking at? An established academic journal? An edited book by established scholarship? An university publishing house? Otherwise, you can only judge the content itself. (Which is difficult to do if you have no extant knowledge to base your evaluation on, but at the bare minimum: do the arguments in the paper even convince you personally or not?)