Of particular interest to prospective hams just finishing their study and preparing for their license exam, there is a great way to get a basic idea of the call letters they will be assigned, at least in the USA. First they must know the FCC region they reside in based on the address they gave on the license application form. The FCC divides the US into ten geographic regions with a numeric designation for each one; a quick reference is found here. For example, the table shows a numeral 5 for Texas so all New Texas licensees will have a 5 in their call sign as originally assigned. By the way, this can be changed later but that will be a future topic.
From here you can refer to the Hamdata FCC info website and look for the most recent sequentially assigned call signs in your numeric region. You can estimate that your assigned prefix (the letters before the number) will likely be the same as the most recent one issued for your region (unless it is at the end of the alphabet already), and your suffix (the letters after the number) will be the next one or two alpha characters subsequent to the most recent one issued for your region. All of this applies, assuming you take your exam in the next two or three weeks. If you test further out, you will need to check the database again to see the current stats.