How can I tell if an online store is legit?

Could you please enlighten me on how to determine the legitimacy of an online store? I'm particularly interested in knowing what kind of factors or signs should I be looking out for? Is there a checklist or a set of criteria that I could follow to ensure the authenticity of the store? Also, what should I do if I have any doubts or suspicions about the store's legitimacy? Could you provide some practical tips on how to protect myself from potential scams or frauds? I'm quite cautious about online shopping and would appreciate your advice on this matter. Thank you in advance for your assistance.