I need some advises and opinions on how to get better manual focus while shooting video on Canon DSLR's ex. T2i, T3i, etc. Since these haven't got any focus indicators or anything like, like for example GH4 with focus peaking while shooting, although it is possible to get that with Magic lantern, I'm interested in native way. The problem is that you have to go from the eye and assume that focus is right, and I often find after importing videos on PC that focus is way off. How can I improve my workflow, and get better focus?
Probably the best thing you can do is get an external monitor. the electronic viewfinders on most DSLRs are too low resolution to be able to judge focus critically. Many of these monitors have built in edge peaking, or focus detection tools as well to make it even easier. Unfortunately to get a decent high resolution external monitor is going to cost the price of the camera again.
I agree with stib's idea of getting an external monitor, and I would also add that getting a follow focus system can help with focus pulling. A good system like this one will allow you to mark certain spots on the "marking disk" (it's a whiteboard-type material, so you can use dry-erase markers over and over again). I usually mark my focus points on the disk before I start shooting (the T2i and T3i both allow you to zoom in digitally to check focus when you're not recording) and then I can just make sure I move the follow focus from mark to mark to nail the focus each time. It's designed to work with cine lenses, but the one I linked to comes with an adapter for regular lenses as well.
This is a considerably cheaper method than getting an external monitor (and can result in smoother focus pulls), but requires a bit more work and a rail system.