Log into the Host or vCenter, click the Host on the left you want to install the VM on then the Summary tab. On the right side will show CPU & Memory usage, and the Datastores show capacity and free capacity. That will give you a quick overview. If you're using Thin Provisioning for disks, there is a potential to overallocate storage so just be cautious there.
Andrew Hancock (VMware vExpert / EE MVE^2)Connect With a MentorVMware and Virtualization ConsultantCommented:
1. disk - check free space on the datastores
2. memory - how much free memory is available on all your hosts
3. cpu - unless you have many VMs across your cluster, this will not be the bottleneck.
if you want to do it scientifically, download the following, and run the trial for FREE, for at least 2 weeks, then run the reports, and it will tell you what resources you have left, to run X VMs.
Regards,
~coolsport00