How to check if nodejs POST request data can be read from stream or received from body?

Question

I have POST api developed in nodejs and expressjs. This post api can get body in two different ways.

1. using normal body object (body parser)
2. Using request stream

On which parameter I can put condition to check to read data from stream or request body.

Code:

app.post('/endpoint', (req, res) => {
    let reqBody = {}
    if( ?? ) // condition to check where data is from stream or from json
    {
        reqBody = req.body
    }
    else
    {
        let data = ""
        // this will be wrapped with promise function
        req.on('data', (buffer) => {
            data += buffer
        })
        req.on('end', (buffer) => {
            data += buffer
            reqBody = parse(buffer)
        })
    }
})

On which parameter I can put condition?