as there will be many IOTA wallets in the future. How do I know which one I can trust. I think it would be easy to implement a code that sends your seed to someone else, right?
You are right: it's easy to implement a wallet to steal user's seed.
If you are a developer, you can:
- write your own wallet and trusting your own code.
- read and understand the code written by a third party and put your trust in it.
If you aren't a developer, you must put your trust in someone else's code. You must educate yourself by reading forums and slack to see what wallets are :
- trusted by the community.
- open source.
- author(s) aren't random unknown people. (it would be suspicious that the only published software of the author being this wallet)
Don't follow a random link on some random ads.
Once you have identified a trusted wallet, download it, double check that you download it from the "official" web site of the author(s). Once downloaded, validate that the checksum of your binary match the checksum announced of the web site.
Additionally you can use the wallet for some time with very small amounts and see if everything works as expected. (note that a smart attacker will also probably wait for a significant amount of funds in your wallet before stealing it)
A wallet should be open source -- this way, it can be vetted by the community to ensure there are no loopholes or trojans.
Aside from that, you should look to the community and determine which wallets are used, for what reasons, where they come from, and their track record to determine which wallet you want to use.