It's not illegal. You must have a 609 certification to buy it.
The only way you know with certainty if the system is properly charged is to use gauges. Even with gauges, you may take less than a full can. Some folks charge with the sight glass, but that can be misleading. Often times the system is overcharged when using this technique. If you overcharge, the interior of your car will not cool well because the high pressure switch will cycle off the compressor. When the system is overcharged, you'll hear/feel the compressor cyling on and off too often.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.