How to tell Service amperage??

I think this might be a complex question, so I expect someone has a complex answer for me.
When looking at a breaker box, how do you tell the service rating or amperage of the incoming service?
Is it an addition of the main breakers? i.e. 50 Amps on each leg would give 100 Amp service and 100 Amp breakers on each leg would indicate 200 Amp service? I don't think that's correct, but need some help clarifying this.

Can one assume that all you have to do to increase your service is install new main breakers? Or does the service from the pole need to be increased in size?

Please don't say "You need to leave this up to a qualified electrician". This is a general question not related to any current project and I am looking for knowledge.

Anyone???
Thanks