Silver

A "silver CD" refers to how the data is transferred to the disc. If the bottom is blue/gold/green/etc (except in some special cases where a commercial release will be a gold CD) it's usually a CD-R, and anyone could make it. There are CD-Rs available with images printed on the artwork side of the disc using special printers, so that alone is not enough to discern whether or not it is a silver CD.

There are several ways to check whether a CD is silver-pressed or CD-R:
1) The most common and obvious way is to look at the bottom -- if it's any color other than silver (and occasionally gold for some special commercial releases), then it's definitely CD-R. But there are some CD-Rs that are silver-ish, so this alone isn't enough to say definitively that it's a silver-pressed CD.

2) Check for a matrix code printed onto the silver side of the CD around the center ring. Silvers will have this, while CD-Rs will have the matrix code printed on the inner-most ring (often made of plastic):

Here's a bit more reading on the difference between the two, although is mostly discussing official CD-R releases, rather than bootlegs:
https://www.discogs.com/forum/thread/344665

CD-R

CD-Rs or Reference CDs are now usually used in place of acetates or test pressings. These allow the artist and producer to hear the work before it is delivered to the label as finished or to provide a quick advance copy of the finished work. CD-Rs can only be recorded one time. CD-Rs are usually packaged in a plain jewel case or sleeve with an insert from the studio containing information regarding the recording. Because of their relatively small numbers, CD-Rs are extremely collectible and valuable.