Usually, people give those away because they don't work very well any more. This might be from dirty, worn, or misaligned playback heads, worn or broken belts, or any number of problems they had (like it kept eating their tapes). But, they either chose to get another VCR instead of fixing the old one or they went to DVD.
Unfortunately, there's really no way you can verify the operation of a VCR at a thrift store or flea market. I would recommend just getting one from someone you know or somewhere that you can verify the operation, or just get a new one. They're really cheap these days. I saw a refurbished one with factory warranty on overstock.com for less than $45.