An easy way is to check the length of the p= value in the DKIM record. If it's around 216 characters, it's likely a 1024-bit key. A 2048-bit key usually has about 388 characters.
If you're going to do this you should also check for k=rsa since other key types will be different lengths. But I'd really recommend something like in the other comment where you base64 decode and parse it with something like openssl.
