Most of the trout in the lakes around the Lower Mainland are stocked. These trout are known as Fraser Valley rainbow trout, at catchable size (~250g in weight, or 10 to 14 inches in length). Unlike hatchery raised salmon, hatchery trout are not marked, so there isn't a physical difference that can be used to distinguish the two populations in a lake. However, as Biff Chan has mentioned, hatchery rainbow trout do have some physical attributes that allow you to identify them more easily. Because they are raised in ponds/pens, the fins of the trout tend to be chewed up due to the amount of space that they can swim in. Their colours are not as appealing as wild trout, the spots are not as prominent and abundant. The flesh is not as coloured, and tends to be pale/creamy in colour. You are allowed to keep any trout in lakes, which are managed differently to rivers where wild trout/salmon are required to be released.
As Rick has mentioned, Catchable rainbow trout in the Lower Mainland do not fight as well as wild fish because of the environment where they are brought up. On the other hand, interior lakes are stocked with trout at a very small size (fingerlings) and by the time they grow to catchable size (several pounds), they are semi-wild and the fight is remarkably strong.