Hi there, I am aware of many websites that claim to be online mastering studios, so you make a better choice here are some tips:
1)Check to see if the studio has mastering grade speakers (not small standard studio monitors) There is no way you can perform professional mastering work on speakers that are incapable of producing deep, accurate bass. Too many bedroom people are playing mastering engineer.
2)Check the credibility of the engineers working history, clients, are they really pro or blagging it ?
3)Ensure you see a picture of the equipment then site suggests they own and use.
4)Avoid text and graphics only based sites that do not list previous clients and do not show photo's show the room they work in.
5)It is always nice if the engineer shows his or her name on the site. Gives you a feeling of confidence and that they are gonna be around for a while and not a here today gone tommorrow affair.
6) Ask for a free sample, this sorts the men from the boys and you can hear the quality on offer.
There are lot of people offering bogus mastering and these tips will certainly keep you on the right path cheers.
online mastering service