The first thing to consider here is whether you trust them, and whether your endorsement will help you and your brand. The second area to consider is the relevance and authority of the site. Is it a sensible website to be linking to, and does it add to your visitors’ experience on your site? The third area to think about is who they link to and from. Using an SEO tool like Majestic SEO you can get a good idea of their link activity so you can make a decision.