"Thanks a lot" in a standard tone usually is merely thanks, and there is no sarcasm. To make "Thanks a lot" sarcastic, it has to be either said in a sarcastic tone AND/OR the 'thanks' contradicts what was just said or done. As an example, if someone hit me I could say "Thanks a lot" and it would be understood as sarcastic whether or not I said it in a standard tone.

When said in a normal tone of voice, "thanks a lot" as the same meaning as "thank you very much" or "thanks much". However, due to the frequency with which it has been used sarcastically, if you intend it seriously, I would perhaps use "thanks very much" to avoid any potential confusion. Have a wonderful day!