Funnily enough, even Sopharma clen bought in a Bulgarian pharmacy and sent for testing was shown to be underdosed by one third.
Virtually everything you need to know about clen has been compiled here:
Clenbuterol focus: Research on Clen & Beta2- Adrenergic Compounds, Protocols
In one trial, 6 / 19 subjects (31.5%) experienced mild muscle cramps (the dosage started at 20 µg twice daily and was titrated up to 40 µg twice daily after a week). So despite widespread anecdotes, it's likely a minority of users that ever experience cramps, not to mind tremors (even rarer) with appropriate dosing. To be fair, it is very likely dehydration and electrolyte balance are the root of these issues and they may be more pressing concerns for resistance training individuals especially following strict dietary regimens.
That is to say, you just cannot verify product authenticity by side effects (it's usually a poor method for verifying quality).