You don't have to ask them point blank, but recall the last political comment that was made by each guest. This may also be a good time to consider whether your guest gets hotheaded when discussing politics, doesn't really have an opinion or seems to be able to gracefully discuss politics with just about anyone.
[header] How to host a dinner party with guests holding opposing political views [title] Know where your guests stand on political issues. [step] Before you formulate your guest list know where each guest stands on politics and whether political issues are a hot topic. [substeps] Identify which side of the aisle each guest falls.