How do I check if a string is a valid md5 or sha1 checksum string

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.