There is a thread on this topic:  https://www.researchgate.net/post/What_other_free_plagiarism-checking_software_do_you_use_other_than_Turnitin#view=578c9d7bf7b67ecec35aa622

Lots of different systems are available (if you want the perfect 'load the paper & get a full report' service, then you have to use the bigger ones). Here is a link to the top 10 free plagiarism checkers: https://elearningindustry.com/top-10-free-plagiarism-detection-tools-for-teachers 

The other thing you can do is put any passages of suspicious text into good old Google - it will pick them up fairly reliably (Google Scholar has an 87%-100% hit rate compared with dedicated academic search facilities.