There aren't too many themes to choose from on the marketplace right now: https://marketplace.magento.com/themes.html?___store=default#q=&idx=m2_cloud_prod_default_products&p...
It can definitely be hard to review themes in advance. You can certainly look for reviews or look for themes from vendors that have other Magento extensions as well as that shows experience with the platform.
Some may offer money back guarantees so that once purchased you can then review the code and see if you're happy with it and if not can request a refund after a short time.
As you mention, some companies may link to a demo site from their own website which would allow you to at least test it in a browser. Foreseeably you could use these demo sites to run them through google pagespeed, mobile friendly test or http://webpagetest.org/ to test for speed.
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I´m planning to create a fashion dropshipping store and i am looking for a pre made template to purchase. My funds are limited, so paying for a custom template is not an option. Is there any way to check code quality regarding performance, responsiveness etc. on a demo site before purchasing? Thank you in advance for your help.