So, you think you're being followed.
Two questions: first, who do you think you are? Are you really so secret and important that someone is prepared to spend time and money watching where you're going?
Second, is there someone out there who very badly wants to know what you're doing, who believes that surveillance is the only way to find out, and who is able to follow you themselves or pay someone else to? Unless such a person or organisation exists, forget it.
But if all that does apply to you, how might you tell you're being followed? Don't bother looking for men with turned-up collars, who peer round corners or keep tying their shoelaces. Real surveillants will never be seen doing anything odd; they are chameleons who strive to look ordinary, matching whatever environment they're in.
f you must check for surveillance, don't keep glancing over your shoulder. Appearing to suspect you're being followed suggests you're doing something to merit it.