By viewing page source.
and not necessarily.... read the forum and you will find many instances of plugin a, b, and c work fine until d is added, then b starts eat RAM and processor time until procwatch kills something to protect your neighbor. Or plugin G is great, but not with pagespeed enabled, etc etc. Talk about that kind of thing in this forum and you will get feedback about what works well and what doesn't, or suggestions to something you missed that performs better.
Think of an office building. You rent space there, if the lights go off, or the ceiling leaks then you call the landlord. On the other hand the landlord doesn't tell you how to run your business or what you should or should not do within your business.... after all you paid your rent and the lights and heat work, why would they tell you how to do your stuff.
Same with dreamhost, you rent space, they keep the server online, what you do in your space is your business.