This post is older than a year. Consider some information might not be accurate anymore.
If you have a Raspberry Pi cluster and a bunch of them, you may have to add or replace some of them. But there is an easy way to check with the shell which model you have. If you query the CPU information you will receive currently 4 CPUs and at the bottom the hardware and revision number.
Query the alpha node in my Pi cluster with Ansible
tan@omega:~$ ansible alpha -u root -m shell -a 'cat /proc/cpuinfo | tail -n 14'
alpha | SUCCESS | rc=0 >>
processor : 3
model name : ARMv7 Processor rev 4 (v7l)
BogoMIPS : 76.80
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm crc32
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 4
Hardware : BCM2709
Revision : a02082
Serial : 000000005a7919f7
Do that for your cluster: 
