I have been asked to show some proof couple of times. I normally show my corp id but credit card with employers name on it has worked, too. As would probably just showing my laptop with employer e-mail account or whatever. My impression has been they are happy with just about anything that proves I'm working for the company whose rate I use and why wouldn't they.

Only in Germany have I been asked to show both corp id and credit card but I wouldn't mind showing them every time I check in.

I actually wonder why they don't ask for proof more often than they do = almost never.