Can transition on an airfoil occur without prior separation? i.e. can transition occur without the presence of a laminar separation bubble? If yes, what would this look like in a pressure distribution?



Here, a clear laminar separation bubble can be seen.



The Reynolds number has been increased by a factor of 10. The transition point is still visible, but it is not so clear anymore whether there is a separation bubble. Is there a possible scenario where a laminar boundary layer transitions without first separating?

EDIT: I'm not asking about how to force transition in XFoil or in real life (using turbulators), but rather if "pure" transition can occur naturally on an airfoil.
can transition occur without the presence of a laminar separation
  bubble?


In short YES.
To elaborate, there are usually 3 mechanisms causing the boundary layer to go turbulent. 


Natural transition  
Forced transition    
Bypass transition


Because the question is about the Natural transition; this is usually caused by sufficiently amplified Tollmien–Schlichting waves. The reason why you see a transition followed by separation in low Reynolds number flow is because in the separated flow, the amplification factor is around 4 times as much as for the attached flow. 


  If yes, what would this look like in a pressure distribution?   





  Here, a clear laminar separation bubble can be seen. The Reynolds
  number has been increased by a factor of 10. The transition point is
  still visible, but it is not so clear anymore whether there is a
  separation bubble. Is there a possible scenario where a laminar
  boundary layer transitions without first separating?


As indicated above, it is possible for a boundary layer flow to make the transition to turbulent without separation. 
N-factor growth rate scales as $\sqrt{RE_\infty}$ for attached laminar flow and for sufficiently enough $RE$ you can easily see the transition undergoing without any separation. 

Also, it's best to inspect the boundary layer parameters through the VPLO menu which gives a clear indication on whether the flow is actually attached or separated.




A very good reference for this is Prof Mark Drela's Flight Vehicle Aerodynamics. - Chapter 4.