Talk with acquaintances who have been there. Be sure you know exactly what is included in the tour price and what you'll have to pay for during the trip. The tour price should reflect the number of meals included. If the tour visits six countries in one week, you'll be on the go from early morning until evening and stay at a different hotel every night. A leisurely tour means staying at least two nights in each hotel.
Some tours use hotels in the suburbs, a streetcar or train ride from downtown.
Others use central hotels. Luxury levels vary widely. Ask the travel agent about feedback on a tour company's track record on tour leaders, and hope for the best. My favorite times to go to Europe are April to early June, or after Labor Day through October. These are shoulder seasons when the weather is usually good and prices are lower than in midsummer, higher than midwinter.