In general looking into anomalies in GA is a good bet - find differences in not-so-important metrics like browser shares, countries, network names, screen sizes, time-of-day, etc. you should find enough data to at least convince yourself. It's hard to fake everything properly.
Try actually looking at server access logs and see if you notice any patterns - UAs (phantomjs?), IPs, ...
Resources:
Good luck and report back. Expose them right here on HN.
- Check IP address/Geo location of visitors. If suddenly India or Pakistan developed an unhealthy interest in your site pages - you'll know that it's all a bogus.
- Check Referrer of each visitor. This could hint you either where visitors are coming from or that these are bots - if suddenly no visitors have any referrers