There are lots of massage parlours in ferangland in Udon Thani, scattered around between the Day and Night complex and Nutty Park, but I do not think that all of them offer a Happy End massage. If you look at the massage parlours, try the ones that clearly have rooms in the back, that are partitioned from each other. Any massage parlour that has the reclining seats or mats in the front of the shop, will not be worth looking at.
So, on to the more discreet looking massage parlours, of which there are several. I would recommend visiting one and see what happens, almost like playing a game I guess.
Get a massage and basically see what happens. If you make decent eye contact, and pass smiles to each other, there is a good chance your girl will ask you if you want a 'happy end', or something similar. Whichever way she chooses to ask, you will be left in no doubt as to what she is saying.
If your girl does not suggest it, you can always ask, but be prepared for a firm no. From what I can gather, some will oblige quite readily, but others may be a little more select, and if they are making enough money, may not wish to do this for you, even for some extra cash. Hand relief is what you will typically be offered, but you may get even luckier under the right circumstances, but that is not a guarantee.
How much you pay is up to you. You may be able to get this service for as little as 100 baht, but you are likely to be asked for more (up to 1000 baht if they think you will pay it!). I would suggest around 250-500 baht, starting at the lower end and haggle. Make sure you do this before she starts to work on you, relax and enjoy.
You can ask in advance, before entering the massage parlour, but you may experience some resistance, and I would not recommend this course of action, it is better to just go along and take your chances as outlined above.
There is one sure fire way of gauging if you are in a massage parlour that offers a little extra. If you going for a foot massage, this will include a leg massage. What you can do is arrive in longer than normal shorts, tight shorts, or trousers. Obviously you will need to remove these in order to receive the leg massage, and as the girls hands reach the inner thigh, the chances are that she is going to brush against your manhood to see if there is any life there. If your trouser snakes does wake up from it's slumber, even just a little, you can bet she will notice and the extra is likely to be offered. Watch her eyes as she massages your legs, and you will see a few discreet glances and your manhood as her hands get closer and closer. If it is not that sort of massage parlour, they are far less likely to be looking.