Assume that you could only see the ball from just after contact (with server's bat). 

For that set of returns, I find it easier to decipher spin of those serves where first bounce is not near table edge (typical short serves, for example). Backspin serves go in a straight, low line from serve contact point to table. Topspin serves sometimes have a bit of curve in the trajectory from serve contact point to table.

However, many servers can serve short effectively with 1st bounce near their table edge (as if they were doing a long serve) - see bogeyhunter's serves in a few of his videos for example. So you pretty much have to look at it's trajectory after ball bounce.

If such serves are hidden, how do you, by looking at ball trajectory, determine whether they are topspin, backspin or no-spin?

A close example of such serve trajectory (even though it is not hidden in this case and not a short serve) is in the following set of serves at 1:45 and then at 2:53 in the video below - if you couldn't see serve action, how would you know which one had backspin on and which one had topspin?:

http://www.youtube.com/watch?v=O4TDQe9ce_M&list=PLF8B6FB47EECC1C0C&index=13" rel="nofollow - http://www.youtube.com/watch?v=O4TDQe9ce_M&list=PLF8B6FB47EECC1C0C&index=13

Yeah, but assume that that won't fly - pretend that it is a refree-d match.

2. If you cant see the stamp, then backspin always stays on the air a little more (top of the 1st bounce) and decelerates (tens of seconds or miliseconds, but enough to judge if you are concentrated) after the 1st bounce (topspin accelerates) because backspin rotation resists gravity a bit compared to topspin which gets reinforced (sorry about my bad english its the best expression i can come up with right now) by gravity

Moreover, backspin is always a slower serve than topspin or in other words a "less lively" one

The difficult thing is to judge between no spin/ light backspin and sideunder/sidetop. Before the hidden serve rule those serves where extremely difficult to recognize especially when the server has practiced enough to give the same curve and placing it in the same spots

Don't agree at all.  I played during the "hidden serve" era, and you had to learn to judge spin by it's trajectory too.  It is a very useful skill to understand how different spin causes the ball to bounce in specific ways.  I've seen some very clever serves where just following the motion can lead you down a bad path.

I am a proponent of starting your serve receive by standing close in to the table such that you can get a very good view of the ball.  It actually makes a difference in being able to see the spin on the ball very early.  Yes, it also helps to be able to focus better on a server's motion to possibly spot the deception in their motion and have a clue as to the spin that is on the ball.

You can actually spot a no-spin serve quite well just by watching the ball closely.  A ball that is spinning is harder to judge just by looking closely.  So, relying on some understanding of how the ball bounces is very helpful with spinny serves.  For instance, a backspin ball will tend to "float" between each bounce.  A topspin ball will tend to "run" when it bounces.  A sidespin ball will tend to curve as it bounces.

Easier said than done, I know.  Practice, practice, practice.  Remember there isn't just one way to return a serve.  But, if you know what spin is on the ball very early, then you've got the advantage on the server.

What, you mean using the illegal rubber sheet with worn and uneven surface?  Ha. Ha. JK! 

You may be able to get some serves back more easily, but the problem with SP is that you'll have a harder time controlling the 3rd ball.  Sorry. 

The forehand rubber is this one (yes, I play with 2 pimples rubber... )

Regarding the 3rd ball I have absolutely no problem with it. Generally my opponent is not able to attack my reception, pushing it back to me (tsutsuki). So I can choose to shoto in angle or forehand smash the ball. 

So, you use LP on your backhand to passively return and rely on your opponents lack of ability to read spin and force a mistake or weak return.  Well, it is a strategy that works to a point.  But, you are handicapped in being able to attack a serve.  And as you say, "generally my opponent is not able to attack", but what happens when they do.  Again, you are at a disadvantage because it is more difficult to control a well-looped ball with SP or LP.

I get how LP or SP can help to cause others to make a mistake.  But, I wouldn't ever recommend someone who is developing as a player to switch to LP or SP to cover up a weakness in their game.

The weakness in my game has always been controlling anything using inverted rubbers. I used them for 20 years, as I said in the other topic.

And as I learned to play with SP in the beginning it is my natural game. I really like fast loops against me. Blocking is what I do best. However, when opponent is able to understand my game and execute slow, spinny lops, I agree with your point. This is the worst ball for a SP player. But in my category (in my region) I can count 1, maybe 2 players capable of playing like that. 

And finally I go to numbers. 20 years with inverted and only 3 golds. Now 6 months with SP and already 3 golds. More efective and more fun, there´s nothing to argue with my results.

I do the same.

what pips are that? and blade