You can get a cheap SPL meter for under $50. It wouldn’t be acceptable for testing industrial legal/regulated loudness, but it would be better than using a totally uncalibrated iPhone.
If you use the same setup to record, and the same distance, you can compare the relative levels.
When I imported the mp3 file in Audacity the sound amplitude is normalized between -1 to 1.
There is a little drop-down arrow at the left of the waveform, and if you click that you will see an option for Waveform (dB). But as Steve said, in an uncalibrated set-up there is no known relationship between dBFS in the digital file and dB SPL (acoustic loudness).
However, if one recording is 6dB louder than the other that 6dB difference is valid in both dBFS and dBSPL. Note that zero dBFS is the “digital maximum”, so most dBFS readings are negative (larger negative numbers are “quieter”). Whereas, zero dBSPL is the threshold of hearing and dBSPL readings are positive.
You can test the peak level by running Effect → Amplify and checking the default (then cancel the effect so you don’t actually change anything). But, the correlation between the peak level and loudness is poor. And if the files are clipped, the peak readings are totally useless.
There is an optional [u]stats.ny[/u] plug-in that can give you RMS & A-Weighted RMS values. These will correlate better with relative loudness.