a=[1,3,4,5,8,10]

a=[1,2,3,4,5,6]

if all(x[i] == x[0]+i for i in range(1, len(x))):
    ...

>>> a = [1,2,3,4,5,6]
>>> a == range(a[0], a[-1] + 1) # in Python3, you need list(range(...))
True
>>> a = [1,3,4,5,8,10]
>>> a == range(a[0], a[-1] + 1)
False

>>> a=[1,2,3,4,5,6]
>>> all(i==j for i,j in enumerate(a, a[0]))
True

>>> a=[1,3,4,5,8,10]
>>>
>>> all(i==j for i,j in enumerate(a, a[0]))
False

def check_sequential(a):
    return set([p - q for p, q in zip(a[1:], a)]) == {1}

print check_sequential([1,3,4,5,8,10])
print check_sequential([1,2,3,4,5])

False
True