No - if music is free - it does not 100% mean it is royalty-free. Most royalty-free music requires you to pay for it, for the continued or one-off privilege to use it! There are also plenty of websites that provide free (cost-wise) and royalty-free music - in which case lucky you!
If it isn't labelled "royalty-free" or something similar, it's best to assume it isn't. However, contacting the Support staff of the website or resource you're browsing the music on, or contacting the artist and/publisher directly, allows you know 100% whether it is royalty-free or not, and also gives you proof in writing in the unfortunate case they change their minds later on. In your case, see if you can find the artists' emails or Facebooks and hit them up, or contact the 'good resources' and ask if they have any way to tell whether their music is royalty-free or if they have an option to only show you royalty-free tunes.
At the end of the day, fair use, copyright and royalty laws are a complicated subject that any new DJ should really take the time to research and get to know, especially if they intend on DJ'ing public, broadcasted gigs or releasing their own mixes and mashups (Especially on SoundCloud, in your case). That's all up to you.
But to answer your question - No, music being free to download isn't necessarily royalty-free, but sometimes it can be, and it's up to you to find out whether it is or not. Better to be safe than sorry!
Best of luck with the DJ'ing - I hope you make it big!