The it is quite easy to calculate a hash checksum of the current block using the '#' command.
Change the block size, seek to the interesting offset and calculate the md5 of it.
$ radare /bin/ls
[0x08048000]> s section._text
[0x08049790]> b section._text_end-section._text
[0x08049790]> #md5
d2994c75adaa58392f953a448de5fba7
In the same way you can also calculate other hashing algorithms that are supported by 'rahash': md4, md5, crc16, crc32, sha1, sha256, sha384, sha512, par, xor, xorpair, mod255, hamdist, entropy, all.
The '#' command can accept a numeric argument to define the length in bytes to be hashed.
[0x08049A80]> #md5 32
9b9012b00ef7a94b5824105b7aaad83b
[0x08049A80]> #md5 64
a71b087d8166c99869c9781e2edcf183
[0x08049A80]> #md5 1024
a933cc94cd705f09a41ecc80c0041def
[0x08049A80]>