Ask literally everyone you can and start early! I asked all my subject teachers, sixth form staff and many other teachers to look at mine, and the pharmacists I did work experience with, and some pharmacists online that were willing. Also TSR has a PS checking thing, it's completely private, only the people that check (who are already at uni or have finished) can see it so you don't have to worry about people copying
Also personally I found it easier to ask one person after another. Starting with family who can just check for spelling and grammar errors, then working up to the people who know the most about the subject and personal statements. When I tried to give a few out at a time for people to check they all came back with different suggestions and it got really confusing, but like I said start early because it can take each person a while to look through it thoroughly. Don't be afraid to disagree with people, the teacher I loved and respected the most actually came back with a lot of suggestions I didn't like, the few changes I did make that she suggested I was told to change back by my more experienced Chemistry teacher