there can be a wide range of 'symptoms' ranging from the obvious like your content has totally changed to the not so obvious like your pages now contain scripts that you didn't put in the pages.
A few years back, my host got hacked and a PHP script was run on many of the directories of a shared server. It went looking for sites with PHP scripts and inserted their script at the top of the page. Totally broke one of my sites, so it was easy to spot and alert the host.
Sometimes it's as simple as a new htaccess being put in a site that redirects traffic someplace else. Basically, there are 1,001+ hacks out there, so there are not really any standard 'signs'.
What makes you suspect a hack/hijack?
oops, just realized I assumed server side hack/hijack - you could have been talking about browser...
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