Check if string is a hash

Question

I'm using SHA-512 to hash my passwords (with a salt ofcourse). I don't think that what I want is possible, but let's ask anyway.

Is there a way to check if a string is a SHA-512 (or another algorithm) hash already?

When a user logs in, I want to do a check on his password. If it's still in plain text, it should get converted to a secure form.

Answer 1

Your task is extremely simple and require no strings checking.

Just compare entered password with stored one first.
If matched - here it is, a plain password. So, you can start conversion process.

Answer 2

As @zerkms already mentioned the string length is the most obvious thing you can test against. Also hashes usually are written in hexadecimal, so it only consists of the digits 0 to 9 and the characters a to f. Or as regular expression

/[0-9a-f]{64}/i