How can one decide to recap an amp when there's no obvious problems yet?
(e.g. no bulging caps, no real sound issues)
I can assume that the sound could change very gradually over time, making detecting the need for recapping harder.
Is there a rule of the thumb for 'modern' amps like e.g. a NAD C320BEE?
30 years is the general rule for electrolytics. Almost all of them will dry up by that time. That's what they do - dry up. The caps won't bulge or give any outward appearance of being bad - they'll just have little to no electrolyte inside them anymore.
If the amp is between 10 and 20 years, then it's probably best to leave it alone. Even with 30 year electronics, you may have to replace all of them before noticing a significant difference. Younger than that, and the work is probably not worth it. JMHO.
Film caps are different. They don't go bad. So if you are wanting to change them, use a much higher quality cap if you expect to hear a difference.
The most obvious place for replacement is if any of the caps are in the signal path. Refer to the schematic for help on that. You can look up many vintage amps on HiFiEngine.com. If you do have some in the signal path, then try replacing them, first. If film, then a Mundorf or Auricap - sometimes even a Wima box cap - may make a big difference. If they're electrolytic, go ahead and try an audio quality electrolytic cap such as Nichicon ES, FG, or KZ, or an Elna Silmic RFS II. Look carefully, though, if your vintage piece of equipment was of high quality, it may already have caps such as those.
I've rebuilt several old Sony tuners, both ES and non-ES. The ES actually had Elna in many signal path positions. The non-ES even had Nichicon ES (not related to the "ES" in Sony ES equipment). In some cases, if the electrolytic cap is in the signal path, you may get good results by bypassing them with a Wima box cap or better still - a Vitamin Q or similar. Typical values for bypass caps are 0.1uf to 0.3uf to get good results.
Good luck!
