How can one verify that meat was slaughtered by a Muslim, especially if the slaughterer does not pray? And does the same ruling apply to restaurants, taking into consideration that I live in a Muslim country like Morocco?

As long as you live in Muslim lands, eat from what is sold in their markets and shops. The default permissibility of their slaughtered animals and the assumption that they were slaughtered according to Islamic law is sufficient for you, as is the fact that whoever professes Islam performs prayers. The default is that only what is permissible to eat is sold in Muslim lands, and you are not obligated to research and investigate who slaughtered it.

It has been stated in the fatwas of the Permanent Committee that the default for a Muslim is goodness, and their slaughtered animals are presumed to be in accordance with Islamic law, and their meat is eaten. It was narrated in a hadith from Aisha (may Allah be pleased with her) that the Prophet (peace be upon him) said to a people who brought meat and it was not known whether Allah's name was mentioned over it or not: "Mention Allah's name yourselves and eat." As for imported meats from abroad, the ruling concerning them is as previously mentioned if they are from animals slaughtered by Muslims.

It should be noted that abandoning prayer out of laziness is not considered disbelief by the majority of scholars, and this is the opinion we give fatwa by.