You would definitely need some kind of reporting tool to be able to perform such assessment. There's very little you could do without a reporting tool really, even if your customer has just a few ESX servers - discovering network and storage topologies, resource allocation etc. could become a serious challenge.

The data those reporting tools produce should give you a good starting point for health research. In most cases, you would be able to see most problems right away from that data, given you have good level of knowledge about VMWare Infrastructure. By the way, some good reporting tools even create comprehensive Visio diagrams of the environment (like network topology), which would make your life tons easier when creating your assessment report. Just those reports alone look very impressive.

Now, in case when customer has issues which come and go, you would need to use some monitoring solution in order to be able to gather and analyze data for extended periods of time.

To start, I suggest that you look at Veeam Reporter tool, it should fit most (if not all) your needs as a VMWare consultant. Just search this forum to find more information about it. By the way, you can license it on per-engagement basis, very good deal there. And in case you need extended monitoring, Veeam has another product for it as well.

Hope this helps.