I look at how these glyphs appear in text strings of what I consider to be the most typical usage.
For instance, the @ symbol most often occurs in email addresses, with lower case letters either side.
While I weight /& as a lower-case glyph, I set its sidebearings (and kerning) for cap acronyms such as “V&A”.

It also helps to study how the sidebearings are handled in solid, well-established typefaces similar in genre to what one is working on.

Thank you both.

Some additional questions:

1. is there something like a "Lorem Ipsum for Symbols"? I wrote some mathematical formulas to set integrals, Sums and so on in their native ecology, also some simple phrases that use astrisks, comma, inversed question marks and so on, but I'm wondering if there is a standard Symbol LIpsum.

2. as far as I gather, there is no standard way to draw the copyright and original symbols - sometimes they are Caps Hight tall, sometimes less, sometimes aligned top and sometimes aligned middle. What determines the position and size? Is it only the designer's decision?

3. I have some trouble understanding how to align some symbols vertically. Which symbols should be centered vertically to the x-hight, which ones - to the Caps hight, when should there be two versions?

4. Are there symbols that should not be italicized? I read something about this in another topic here, I'm wondering what are the rules in this respect.

5. as far as I know, there are symbols that don't respect the overall rules of the font and keep their special stem width and size because they are used very seldom, like a typographic dialect with it's one rules. Does anyone have any information on this?

6. even if there are no standard rules, are there certain principles of construction, like "if you design the copyright this way, you should design the partial differential that way, and the eth so-and-so" Besides, of course, basic features like stem thickness and overall size.

I am aware this are very very minute and trifle matters, but I believe they nonetheless have their importance for a well-made font.

Thank you for your time.

2) What's the "original" symbol? Do you mean the registered trademark? The standard for copyright is to be full-size and centered on the cap height. For registered, it is to be a superscript position. Plenty of people get these wrong, but those are the correct choices for almost all fonts.

3) Which specific symbols do you mean?

4) Non-italicization is open to debate, for sure. But I certainly have a list of symbols to not italicize, borrowed from work I did at Adobe.

5) That's a choice by the designer or foundry, but in general I don't see much point in it. Why put in bogus character support? I'd only do that if I had a specific client or customer need for it.

6) I don't think I have ever seen such guidance thoroughly enunciated. But you might check Microsoft's character design guidelines, or Karen Cheng's book. Maybe the old Monotype type design standards have it, I'd have to look. (That is internal and unpublished Monotype material. Assuming I'm not confusing Mono and Lino here.)

Note that the superscript registered is also smaller than the copyright symbol. It's not just raised.

I have just read the Microsoft character design guidelines and they pretty much answer my questions. Thank you for the information on the registered and copyright symbols, I find it very useful.
I'm also currently following Shinn's advice and studying glyphs and side bearings in Helvetica.

The @ symbol varies a lot. In Centaur, which I use often, it is very large compared to the lower case letters that usual accompany it (email address). At first I found it too overwhelming but now I have grown quite to like it. It does accentuate that there is an email address. Have a look at it.

And I would like to know why the ℗ (music recording publication/copyright symbol) is missing from so many fonts.

Because it’s not in the standard encodings.
http://www.unicode.org/charts/PDF/U2100.pdf

ah, thanks