Base what?

Aside from Base64, there are many different ways of encoding data. For example, there is even hexadecimal representation or even Base85.

Why others?

Some CTF challenges are proud with being unique and may throw one of these to confuse the ill-prepared contestant. Malicious actors may use obscure encodings to hide payloads and mask illegitimate traffic.

Recognizing them

The ability to recognize different encodings fortunately is mostly straight forward.

If it is a string of 1 and 0 like 01100010 01101001 01101110, it is likely binary.

If it a string containing digits and a-f like 6b 69 74 74 65 6e 73, it is likely hex.

If it contains a long string of alpha-numerical characters (a-z,A-z,0-9) and no spaces, it is likely to be a type of Radix encoding such as Base32.

Finally, all the forms of encoding mentioned above can also be decoded at CyberChef.