How do you enforce handwriting standards for homework assignments as a TA?
My approach is simple- homework is submitted as a typed .pdf file, no exceptions.  Homework is submitted online through the course management system (Instructure's Canvas in our case), which enforces the required file extension.  

Yes, this means that my students need to learn how to produce typed mathematical equations.  They're welcome to use LaTeX (probably the best choice in the long term), Google Docs (which has a pretty intuitive system for math formulas that's adequate for most purposes), MS Word (which has an awfully complicated and hard to use equation editor), Apple Pages, or whatever software they like, but they have to convert the output to pdf so that I don't have to deal with all of these different word processing software packages.