Yes, when a session is over you can get the RTP for that particular session, or can request via e-mail your overall RTP for that site since your first spin.
Now this is quite subjective - some sites like 32red and 3dice can give you this via live chat because they are clued up on it and get asked it quite a lot, and are obviously trained to access the details and provide them. 3dice are particularly generous with their stats.
Some RTG sites will give you a load of mathematically impossible nonsense because they either don't understand it or can't assess it correctly.
On most MG sites you will need to e-mail them as online chat don't know much about it.
All depends on the casino - you'll soon get used to via trial-and-error which are most conversant with the terms session RTP and overall RTP on your account.