In Sherlock Holmes stories, business cards are used as means of identification. What means of identification were available in the 1890s and what were their pros and cons?

I would start by looking at the answers in the Identity Theft section of the subreddit FAQ, with special attention to the answers by /u/Georgy_K_Zhukov and /u/peripatos. Zhukov's answer is close to your question in time, albeit on the other side of the Atlantic Ocean from 221B Baker St.