Other anime streaming sites are blocking my IP because I'm Iranian while anime1.com isn't. I wanted to know if it's legal to watch/stream anime from this site. Thanks.
You might want to see this post: How can I tell if a site is Legal? It lists legal sites for streaming anime and reading/buying manga.
Is anime1.com legal? Nope. An immediate indicator that a site is illegal is if it's laced with ads that are usually irrelevant to anime/manga in general. anime1.com's anime pages are almost filled up with ads and clicking on anime episode links results to pop-ups.
'You can also check the site information to confirm if they have a valid certificate and connection type,' as mentioned by @Rumplestiltskin. If a green padlock is present, that site is most likely secure and a 'Connection is secure' message will pop out whenever you click on it. This helps you avoid 'really dangerous anime-scam sites.' anime1.com does not have this so be wary of sites like these.
Clarification: I just want to clarify something since people seem to misunderstand. The check for valid certificate is not a way to check for a site's legality. It is just for security purposes. There are many ways to check for a site's security but that, I think, is outside the scope of the question.
You can also check by looking at the anime selection and whether they could have a license for them. Quite a few anime are only licensed to a single site.
For example as of this writing Violet Evergarden and Devilman Crybaby are Netflix exclusives and Evangelion is not licensed to any streaming site . If any other site provides them then they are pirated.
Another method is by the player. A lot of pirate sites will outsource the actual video hosting by using embedded video players from other sites. Some have multiple such hosting sites that you can switch between.
If a streaming site is offering you copyrighted material for free, you can be confident that site is not operating legally.
However, watching/streaming anime (or other material) from a site is considered legal in many countries, even if the site itself is not legal. Whether this applies to your country would be better asked at law.stackexchange.com.
Despite of what was suggested in the other answer, checking if the site has a valid certificate and secure connection is bad and even dangerous advice, as nowadays any site can obtain those for free in just a few minutes with zero background checks through open certificate authorities like Let's Encrypt.