I've played in games where I've gone back on agreements and I've played games with players who played a very trust-worthy game in one and then not so much in another.
It's all part of the game.
Now, some steps to help mitigate somebody backstabbing you:
When I make an agreement with another player, I almost always ask for a demilitarized border. This way, if they plan to backstab you, you have some time to react to subs moving back towards you.
Keep your communication and chatter going. I'm generally very open with my movements, plans, and what I'm thinking with people I'm allied with. This humanizes you and also gives your ally a chance to share details on what their thinking. Warning signs can go off if they become less social in the game.
To show trust, I usually make the first move, but I always have it planned so if they move counter to the agreed movements, then I have an opportunity to respond before I'm all in on whatever movements we agreed too.
"If we don't know what we are doing, the enemy certainly can't anticipate our future actions." - Colonel Goodhead