I do have some experience with frontal lobe dementia, and I am happy to share what I know.
However, I am not a medical professional, and for that reason I would like to refer you to Alzheimers UK where it is explained in detail by people who are in a position to give advice.
Frontal Lobe Dementia starts at a much earlier age than the dementia that we usually hear about which usually affects the older generation. People in their 40s are most often affected by Frontal Lobe Dementia.
It has a lot of similarities with OCD and depression so it is easy to mistake at first, but it does become apparent after some time that something is not 'right'.
My family member started to withdraw from people, got confused when making a decision, even when it was a relatively simple one, and started to talk to, and about, people with a severe lack of tact.
It is embarrassing to be with a person who starts to do that when it is not apparent at the time that there is a medical reason for it.
After that his speech started to become a problem, he seemed to struggle to form words and started tapping his fingers on the table, his leg, etc. This is something that annoys me generally anyway, but by this time we realised there was a problem so obviously it was forgivable.
It is not an easy thing to see a loved one go through, but there is help and support out there luckily.