Determine if collection is of type IEnumerable<T>

If you mean the collection, then just as:

var asEnumerable = i as IEnumerable<int>;
if(asEnumerable != null) { ... }

However, I'm assuming (from the example) that you have a Type:

The object will never be "of" type IEnumerable<int> - but it might implement it; I would expect that:

if(typeof(IEnumerable<int>).IsAssignableFrom(type)) {...}

would do. If you don't know the T (int in the above), then check all the implemented interfaces:

static Type GetEnumerableType(Type type) {
    if (type.IsInterface && type.GetGenericTypeDefinition() == typeof(IEnumerable<>))
        return type.GetGenericArguments()[0];
    foreach (Type intType in type.GetInterfaces()) {
        if (intType.IsGenericType
            && intType.GetGenericTypeDefinition() == typeof(IEnumerable<>)) {
            return intType.GetGenericArguments()[0];
        }
    }
    return null;
}

and call:

Type t = GetEnumerableType(type);

if this is null, it isn't IEnumerable<T> for any T - otherwise check t.

Since IEnumerable<T> inherit IEnumerable (non generic) and if you don't need to know when a type is just IEnumerable and not IEnumerable<T> then you can use:

if (typeof(IEnumerable).IsAssignableFrom(srcType))

Same technique as Marc's answer, but Linqier:

namespace NS
{
    class Program
    {
        static IEnumerable<int> GetInts()
        {
            yield return 1;
        }

        static void Main()
        {
            var i = GetInts();
            var type = i.GetType();
            var isEnumerableOfT = type.GetInterfaces()
                .Any(ti => ti.IsGenericType
                     && ti.GetGenericTypeDefinition() == typeof(IEnumerable<>));
            Console.WriteLine(isEnumerableOfT);
        }
    }
}