Question: I don't want to calculate a file's checksum, just to know if a given string is a valid checksum

Answer: 
SHA1 verifier:

```
public boolean isValidSHA1(String s) {
    return s.matches("^[a-fA-F0-9]{40}$");
}

```

MD5 verifier:

```
public boolean isValidMD5(String s) {
    return s.matches("^[a-fA-F0-9]{32}$");
}

```
Any 160-bit sequence is a possible SHA1 hash. Any 128-bit sequence is a possible MD5 hash.

If you're looking at the hex string representations of them, then a sha1 will look like 40 hexadecimal digits, and an md5 will look like 32 hexadecimal digits.