I tell them I will.
I am not sure if there is a God or not so I guess that puts me in the agnostic category if there is a need to define by "category".
The thing is, I dont know anything about how all of that works (and neither does anyone else).
If the mere act of hearing their request and answering in the affirmative gives them comfort or is soothing? I will do it.
For me it appears rather like telling a bed time story to a small child who cant sleep. Its soothing and comforting whether its true or not.
I can at least provide that.
My thinking is that if someone asks me to pray for them it is either perfunctory (like have a nice day - doesnt really need an answer) or they are really afraid/desperate and I can meet their current need without invoking my personal views.