Most churches identify themselves and their denomination pretty clearly. If a church has a sign outside it generally gives its full name, which usually includes the name of the denomination if it has one. For example a Catholic church called "Saint Anthony's" will usually have a sign saying "Saint Anthony's Catholic Church" outside, and the same for larger denominations like Anglican, Methodist, Lutheran etc.
The same is true of a church website, which generally gives the full name including denomination. It may also give the denomination under a page like "What we believe".
A lot of churches will identify as "Baptist", which probably tells you what you want to know but doesn't identify the denomination exactly, as there are lots of different flavours of Baptist. Their website probably identifies which association of Baptist churches they belong to, if any. Similarly for "Pentecostal".
If a church doesn't identify a denomination then it may be "non-denominational" which means it doesn't belong to any larger organization (though some non-denominational churches form themselves into groups). A church that calls itself "community church" or "Evangelical church" is often non-denominational.
Any church that doesn't identify as Catholic or Orthodox may usually be considered Protestant for practical purposes, although there are some churches that do not identify as any of those three.