Through some relatives I got a Cannondale R700 2.8 bike for free, but it has a dent in the frame, and I heard from some people that it could be serious issue. So my question is, is it safe to ride this bike or how can I tell so, if it's not possible to tell from pictures? What are the risks of using this bike?
With all frames, that much of a dent increases the risk of a failure eventually. If that were an ultra-light frame with very thin tubes, it might be better to retire that now, but an earlier model Cannondale probably doesn't qualify.
You could certainly retire the frame if you want, but another option is to keep an eye on the dent and to watch for cracks propagating from the area. This does mean regular visual inspection, not just check it once in a while at irregular intervals. It might also be good to monitor how the frame feels when riding, especially when out of the saddle or going over bumps. It's worth stating that a failure might be sudden, and not gradual, so periodic inspection like this isn't guaranteed to catch the frame before it fails entirely. If this were my bike, I’d choose watchful waiting.