Deceleration points are indicated on the ND by a small circle (but unlike things such as T/D are not labelled)...it’s in the FCOM under ND symbology.
As for the OPs initial question..”Say on a STAR with a speed constraint, when will the FMC command a reduction of airspeed? ”... I’m not sure what info the OP wants,if you have a constraint to cross a way point at e.g. 250 knots the FMC will command the decel at a suitable point to achieve the 250 knots at the waypoint. How it calculates the point, what the algorithm is, I have no idea. It is not in any document I have seen and of course on the line you just keep an eye on it to make sure it does what it should....no point in worrying about how it works it out.