A mastering engineer is someone who is charged with the task of creating an album out of a number of tracks and mastering the audio itself, thereby improving its quality. Not all mastering engineers are the same which is why there are number of things to consider when hiring someone for their mastering services.
First, you could limit your search to mastering engineers who offer a free test master to all new clients. Even if it’s the most respected and decorated engineer in the world, I wouldn’t trust my audio with them before hearing what it’s going to sound like, especially for the cost which is likely associated with hiring the most decorated mastering engineer in the world.
Any reputable engineer will offer a free test master so that you can experience how much better or hopefully not worse your audio will sound in their hands. If they do not offer a free test master to new clients and that’s a red flag which likely suggests that you could have problems down the road if you went with them.
Second, you should consider the cost associated with that mastering engineer. Some engineers charge by the hour or the track whereas others will give you an estimate for the entire project. It’s obviously best to get a quote upfront from that engineer so that you know exactly how much you can expect to pay.
By the same token, you should and need to clearly lay out exactly what you want from that mastering engineer. Some artists these days are concerned with CD or vinyl replication and only want individual tracks to be mastered for online distribution. There are obviously great number more costs associated with creating an entire album file so you need to clearly lay out all of this initially and that will of course dictate how much the entire project will cost.
The engineer will likely have some requirements for you, as well, in terms of the kinds of formats which your audio needs to be in before they will work on it. Many engineers prefer not to or will abstain from working on certain inferior file formats such as MP3 when a wave file is much more preferred.
Ultimately you want to have a good feel and feel like you have a good read on your mastering engineer moving forward with him or her that you know that if there are any issues which need to be addressed you can access them quickly and easily. This is especially helpful in the case of needing to their work after you have receive it back. This is just something else which you should establish with your mastering engineer early on in the process so that there are no surprises moving forward with them.