Unspoken (not body) Language

So think of a time when someone offered to do something for you.
Did they really want to do it? or did they feel obligated to offer?
How can you find out?

Proper social etiquette likely dictates you should  politely decline while giving them the possibility of helping you anyway.

"Oh, it's alright, I don't want to inconvenience you." or something like that.

The reason you do this is to give them complete control over whether or not they help you.
When they offer to do something, they are putting themselves on the line, they are putting their short term fate in your hands. By declining politely, like in the example, you let them off the hook.

They will reply something like "alright" or "are you sure?"
or
"no its no trouble really" or "come on, its fine"

The first two are him retracting his offer. Yes, even the are you sure one.
The second two are him overriding you presumed hesitation presumably because he genuinely wants to assist you.

Rule of thumb being;

POLITELY decline offers of assistance. If they INSIST, then you are good to, thank them for their offer and allow them to help you. If they do ANYTHING besides insist, even ask if you're sure, etc, you move on and deal with it alone. That is, assuming you don't want to inconvenience them... or if you're not desperate.