FHA loans are typically the only loans these days that can be assumed. So you'd have to find someone selling who currently has an FHA loan.
Also, in order for you to assume the loan, the holder of their loan would need to qualify you for that loan. They generally use the same principles in qualifying you as if you applied for a loan on your own.
In addition, you'd probably have to come up with a larger down payment because when you assume a loan you are taking over only the balance owed and in most cased the seller is going to have some equity in the home.
For example, if they are asking $100,000 for the home and they owe $80,000 on the mortgage, then if you assumed the loan you'd have to put the remaining $20,000 down as a down payment. In that case, you'd probably be better off securing your own financing at a greater amount.
Make sense? Hope this helps!