Assuming that you know the name of the content, you can retrieve the xcontentid using the findByProperties web service which is included in the content package of xcontents. Below you can find the body to be included in the request; within the searchKey parameter make sure to include the name of the content. You will find the xcontentId within the response.

{
    "client": {
        "clientId": ""
    },
    "criteria": {
        "locale": "",
        "textSearch": {
            "searchKey": ""
        },
        "offset": 0,
        "numberOfresults": 50
    }
}

Note: This method can be used also to get the categoryId of a folder (assuming you know the folder's name) or the groupId of a group (assuming you know the group's name).