Coping with high visitor rate

I have been asked by a client to ensure a MODX site I am building is able to handle thousands of visitors (not sure weather they are concurrent). How do you test (hopefully with open source tools) for this and ensure your site will not fall over. Is it best to replicate the server and use some sort of load handling.

The site is hosted on a CENTOS WHM/Cpanel vps using the lattest version of MODX.

Any advice welcome.

Thanks in advance
Glyn

We have build an Product-Launch Page in MODx which had about 150.000 clicks within the first hour, and sold about 2500 Products inside an MODx custom Shop.

We did some stages of Testing:

First "ab" is an good source for massive single Site Requesting. Its avaible on max OSX and Linux Servers most times by default. And you can run tests on the commandline via

https://httpd.apache.org/docs/2.4/programs/ab.html

After that we did some free Online Load-Tests via
https://loadimpact.com/

At least we spend some credits for some big Last tests and hoped the best ,)

Many thanks @michael engel. I am aware of ab and been testing using it, however I have had a number of issue trying to get it to work https. load impact.com looking. Will check it out.