You can use isinstance()
:
>>> from mako.template import Template
>>> print Template("${isinstance(a, int)}").render(a=1)
True
>>> print Template("${isinstance(a, list)}").render(a=[1,2,3,4])
True
UPD. Here's the usage inside if/else/endif:
from mako.template import Template
t = Template("""
% if isinstance(a, int):
I'm an int
% else:
I'm a list
% endif
""")
print t.render(a=1) # prints "I'm an int"
print t.render(a=[1,2,3,4]) # prints "I'm a list"