Hi,
this is a question for who has experience in RouterOS+MPLS 
if I have a VPLS tunnel and I do a ping through the tunnel (using the Dont Fragment flag):
ping x.x.x.x -l 1472 -f
where x.x.x.x is an IP on the other end of the tunnel (I use 1472 because 20 bytes is for TCP + 8 bytes for ICMP = 28 bytes of overhead)
If the ping works and it is not fragmented, does it mean that MPLS+VPLS works properly and the MTU of the physical interfaces are ok?
Not really, it will still appear to work even if
:mpls interface
have an MTU that is set too high. It will just break when you send a labelled packet which is too big for a router (:mpls interface) or interface (:interface) in LSP.
thank you sten. Can you suggest me a way that works for testing VPLS?
Have you tried stacking vlans over and over, putting an IP on the innermost vlan and ping large packets to the router on the other side through the VPLS? It would require a similar setup on the other side to get a response though. This works around the IP MTU quirks in RouterOS and you can add as many as you want in increments of MPLS label size. You could potentially use the method to monitor inside the VPLS.