Unzip the apk and look through the filesystem. Some malware will be obvious.

Install the APK on an android development vm and use it for a bit. Maybe give it a week to start showing ads and stuff. Maybe try capturing network traffic and try to determine if it's legit.

If you don't need network access use netguard or some other local firewall to disable network traffic for the app and just use it normally

Back up your important stuff and be ready to wipe your device if you notice any bad behavior. Of course some malware doesn't show itself at all, so you may never know.