When buying a weight gain supplement there are two main things to take into consideration: the quality of ingredients and the ratio of carbohydrates to protein.
First of all the quality of the ingredients - for the carbohydrates you should generally (unless you're a REAL hard gainer) be looking for low GI carbohydrates such as oats and barley, and avoiding sugary ingredients - dextrose, fructose - in large amounts. With regards the protein part of a weight gainer most brands opt for a protein blend rather than a single-ingredient. This is because this type of supplement is used to replace food meals and you should try to get a varied protein intake as you do with foods. That said highly bioavailable protein sources such as whey protein, egg protein and milk protein should feature in any high quality weight gainer.
The second thing to look out for is the ratio of carbohydrates to protein which range from 1:1 where for every gram of protein you get a gram of carbohydrates to match it, to as much as 6:1 in favour of carbohydrates. Neither of these ratios (or anywhere inbetween) make a product bad, but if you are looking to control your carbohydrate intake then picking a weight gainer with a lower ratio would be advisable.
As always if you have any questions please get in touch and we will advise on which weight gainer supplement is suitable for you.